handle script load errors

[tomwayson]

resolves #14

[davetimmins]

Looks like script.removeEventListener('error', onScriptError, false); should be called when there is no error too i.e. in the script.onload body

[tomwayson]

I think you are right @davetimmins, but am not sure. And here I thought I was going above and beyond by removing a listener for an event that should only ever happen once anyway.

As I mention in #14 (comment), I'm about to push a promise-based branch that uses addEventListener entirely instead of onload and onerror. I'll look into having the 'load' callbacks remove the 'error' callbacks and vice versa, or at the very least ping you on the PR for suggestions.

[tomwayson]

FYI - I didn't add the change (yet) to #48 to "cross remove" the load/error listeners, but it would go hereish. Seems like I'd need to replace those functions w/ a single function that added then removed both listeners, but right now those functions are called by the deprecated functions, so it would be easier once those are removed.

[tomwayson]

FYI - I've addressed this in 2023d6c

I couldn't figure out how to "cross remove" them w/o moving them into a single function, there's a bit of a 🐔 and 🥚 situation, but that will be easier after removing bootstrap() and dojoRequire() and there's no longer any need to keep them as individual functions.