How can a get good error messages about which parameters I was using when the test failed?

merchant_account_spec.rb

require 'spec_helper'

describe MerchantAccount do
  it "can find an appropriate merchant" do
    {
      'GND130415A'                        => 'GRAZ_1',
      'MIS130417A'                        => 'GRAZ_2',
      'ROA130417A'                        => 'GRAZ_2',
      'LON130417A'                        => 'GRAZ_3',
    }.each do |tour_code, expected_merchant_account_descriptor|
      event = Event.find_by_name(tour_code)
      ma = MerchantAccount.find_by_event_and_product(event, nil)
      ma.descriptor.should == expected_merchant_account_descriptor
    end
  end
end

Meaning you want it to say something like "GND130415A doesn't match GRAZ_1, got FOO instead" or something?

The best (only?) way is to write a custom matcher. You end up specifying your own error strings on failures.

assert_equal expected_merchant_account_descriptor, ma.descriptor, "#{tour_code} doesn't match #{expected_merchant_account_descriptor}, got #{ma.descriptor} instead"