should add useful message to ArgumentError

[jbodah]

Currently I get:

blah#test_0001_test_updates_processor:
ArgumentError: ArgumentError
    /Users/jbodah/repos/cg-ruby-sem/test/recorder.rb:177:in `credentials'
    /Users/jbodah/repos/cg-ruby-sem/test/recorder.rb:152:in `do_record'
    /Users/jbodah/repos/cg-ruby-sem/test/recorder.rb:34:in `block in record!'
    /Users/jbodah/repos/cg-ruby-sem/test/recorder.rb:50:in `block in freeze'