Notes-to-Twitter

deploy-succeeded.js

// package.json shown below to double check nothing is missing
import fetch from 'node-fetch'
import dotenv from 'dotenv'
import Twitter from 'twitter'
import { AllHtmlEntities as Entities } from 'html-entities'

dotenv.config()

// URL of notes JSON feed
// output of file shown below
const NOTES_URL = 'https://www.bridgestew.com/notes.json'

// Configure Twitter API Client
// Have double checked values in .env for accuracy
const twitter = new Twitter({
    consumer_key: process.env.TWITTER_CONSUMER_KEY,
    consumer_secret: process.env.TWITTER_CONSUMER_SECRET,
    access_token_key: process.env.TWITTER_ACCESS_TOKEN_KEY,
    access_token_secret: process.env.TWITTER_ACCESS_TOKEN_SECRET
})

// Helper Function to return unknown errors
const handleError = err => ({
    statusCode: 422,
    body: String(err)
})

// Helper Function to return function status
const status = (code, msg) => {
    console.log(msg)
    return {
        statusCode: code,
        body: msg
    }
}

// Check exisiting notes
const processNotes = async notes => {
    if (!notes.length) {
        return status(404, 'No notes found to process.')
    }

    // assume the last note is not yet syndicated
    const latestNote = notes[0]
    if (!latestNote.syndicate) {
        return status(
            400,
            'Latest note has disabled syndication. No action taken.'
        )
}

    try {
        // check twitter for any tweets containing note URL.
        // if there are none, publish it.
        const q = await twitter.get('search/tweets', { q: latestNote.url })
        if (q.statuses && q.statuses.length === 0) {
            return publishNote(latestNote)
        } else {
            return status(
                400,
                'Latest note was already syndicated. No action taken.'
            )
        }
    } catch (err) {
        return handleError(err)
    }
}

// Prepare the content string for tweet format
const prepareStatusText = note => {
    const maxLength = 280 - 3 - 1 - 23 - 20
    const entities = new Entities()

    // strip html tags and decode entities
    let text = note.content.trim().replace(/<[^>]+>/g, '')
    text = entities.decode(text)

    // truncate note text if its too long for a tweet.
    if (text.length > maxLength) {
        text = text.substring(0, maxLength) + '...'
    }

    // include the note url at the end;
    text = text + ' ' + note.url
    return text
}

// Push a new note to Twitter
const publishNote = async note => {
    try {
        const statusText = prepareStatusText(note)
        const tweet = await twitter.post('statuses/update', {
            status: statusText
        })
        if (tweet) {
            return status(
                200,
                `Note ${note.date} successfully posted to Twitter.`
            )
        } else {
            return status(422, 'Error posting to Twitter API.')
        }
    } catch (err) {
        return handleError(err)
    }
}

// Main Lambda Function Handler
exports.handler = async () => {
    // Fetch the list of published notes to work on,
    // then process them to check if an action is necessary
    return fetch(NOTES_URL)
        .then(response => response.json())
        .then(processNotes)
        .catch(handleError)
}

netlify.toml

[build]
  publish = "_site"
  command = "npm build trigger"
  functions = "lambda"

notes.json

[
    {
        "id": 1,
        "date": "Fri Feb 08 2019 17:45:05 GMT-0500 (Eastern Standard Time)",
        "url": "https://www.bridgestew.com/notes/2019-02-08-22-45/",
        "content": "I've been trying to follow along with Max's code examples for how to get webmention sends working with Twitter. Still working out kinks.",
        "syndicate": true
    }
]

package.json

// The pertinent parts
{
    "scripts": {
    "serve": "gulp watch & eleventy --serve",
    "build": "npx eleventy",
    "watch": "npx eleventy --watch",
    "debug": "DEBUG=* npx eleventy",
    "trigger": "eleventy & npm run build:lambda",
    "build:lambda": "netlify-lambda build functions"
  },
  "devDependencies": {
    "@11ty/eleventy": "^0.7.1",
    "@11ty/eleventy-plugin-rss": "^1.0.3",
    "@11ty/eleventy-plugin-syntaxhighlight": "^2.0.0",
    "acorn": "^6.0.7",
    "eleventy-plugin-reading-time": "^0.0.1",
    "luxon": "^1.9.0",
    "markdown-it": "^8.4.1",
    "markdown-it-abbr": "^1.0.4",
    "markdown-it-anchor": "^5.0.2",
    "markdown-it-attrs": "^2.3.2"
  },
  "dependencies": {
    "clean-css": "^4.2.1",
    "dotenv": "^6.2.0",
    "gulp": "^4.0.0",
    "gulp-concat": "^2.6.1",
    "gulp-sass": "^4.0.1",
    "gulp-uglify": "^3.0.1",
    "html-entities": "^1.2.1",
    "html-minifier": "^3.5.21",
    "netlify-lambda": "^1.3.3",
    "node-fetch": "^2.3.0",
    "sanitize-html": "^1.20.0",
    "send-webmention": "^2.0.0",
    "sitemap-generator": "^8.3.2",
    "twitter": "^1.7.1",
    "uglify-js": "^3.4.9"
  }
}

tried running your function in my local setup and it seems to work.
Could you try to expand the handleError function to:

const handleError = err => {
    console.error(err)
    const message = Array.isArray(err) ? err[0].message : err.message
    return {
        statusCode: 422,
        body: String(message)
    }
}

that should give you a more verbose output on Netlify's function console.