Example of using a with statement to wrap your script so that it will notify with an email on success or failure. The error is then re-raised, so whatever is running the original tool will see it. The mail section was taken from the snipped provided at http://gis.stackexchange.com/a/103394/150

script_mail_wrapper.py

import smtplib, os
from email.MIMEMultipart import MIMEMultipart
from email.MIMEBase import MIMEBase
from email.MIMEText import MIMEText
from email import Encoders
import traceback
import textwrap
from datetime import datetime
from StringIO import StringIO

def mail(to, subject, text, gmail_user, gmail_pwd):
    msg = MIMEMultipart()

    msg['From'] = gmail_user
    msg['To'] = to
    msg['Subject'] = subject

    # Attaches a text message to the e-mail
    msg.attach(MIMEText(text))

    mailServer =smtplib.SMTP("smtp.gmail.com", 587)
    mailServer.ehlo()
    mailServer.starttls()
    mailServer.ehlo()
    mailServer.login(gmail_user, gmail_pwd)
    mailServer.sendmail(gmail_user, to, msg.as_string())
    mailServer.close()
    
class ScriptWrapper(object):
    def __init__(self, tool_name, msg_to, gmail_user, gmail_pwd):
        """
        Wrap a tool to send an email on success or failure
        """
        self.tool_name = tool_name
        self.msg_to = msg_to
        self.gmail_user =  gmail_user
        self.gmail_pwd = gmail_pwd
    
    def __enter__(self):
        return #We don't pass back any variables to the with
    
    def __exit__(self, exc_type, exc_val, exc_tb):
        #When the with is ended it comes through here. The exc_type, exc_val, and exc_tb variables are only populated on an error
        if exc_type: #error in execution
            tb = StringIO()
            traceback.print_tb(exc_tb, file=tb)
            text = textwrap.dedent("""\
            Tool '{tool_name}' at {time} failed with the following error:

            {traceback}
            {error_type}: {error}
            """).format(
                tool_name=self.tool_name,
                time=datetime.now().isoformat(),
                traceback=tb.getvalue(),
                error=exc_val,
                error_type=str(exc_type.__name__)
            )
            subject = "{tool_name} failed with error {error}".format(
                tool_name=self.tool_name,
                error=exc_val
            )
        else: #tool succeeded
            text = textwrap.dedent("""\
            Tool '{tool_name}' suceeded at {time}.
            """).format(
                tool_name=self.tool_name,
                time=datetime.now().isoformat()
            )
            subject = "{tool_name} succeeded".format(tool_name=self.tool_name)
            
        mail(self.msg_to, subject, text, self.gmail_user, self.gmail_pwd)
        
        return False #Still want to throw the error so ArcGIS shows it

your_tool.py

import arcpy
import script_mail_wrapper


def main():
    #Your tool
    pass


if __name__ == "__main__":
    gmail_user = arcpy.GetParameterAsText(0)
    gmail_pwd = arcpy.GetParameterAsText(1)

    #wraps your tool to send an email on success or failure
    with script_mail_wrapper.ScriptWrapper("Tool Name", "[email protected]", gmail_user, gmail_pwd):
        main()