Reuse error functions

[jugglinmike]

This builds on gh-40 by also re-using Test262's definition of the $ERROR function.

@bterlson What is Test262's "API"? I can see two possible interpretations:

1. Test262 test files rely on the contents of an opaque file named sta.js (which may change at any time)
2. Test262 test files rely on global $ERROR and Test262Error functions

Although this pull request assumes #1 is true, I'm starting to think that #2 is more realistic. If that's the case, this approach should still work (just with a little more modification).

[bterlson]

I see #2 as the real case - runners have to provide the definitions for these functions because they might change depending on environment. I'm surprised tests pass with this PR...

[jugglinmike]

I'm surprised tests pass with this PR...

They pass because none of the currently-implemented runners require custom definitions of $ERROR or Test262Error. Unlike $LOG and $DONE, I can't think of a case where $ERROR or Test262Error would need to be configured. Do you have any thoughts on that?

The alternative I had in mind is to attach the definitions for these objects as properties on the base Runner prototype--then sub-classes could over-ride if necessary. Does that sound good to you?

[jugglinmike]

I've pushed up another commit to demonstrate the change I was describing in my previous comment.

[jugglinmike]

Documentation will also need to be updated, but I'll hold off on that until I hear back about the code change.

[bterlson]

My original thinking was that $ERROR could do host-specific error reporting but I guess this is not required if $DONE received the error and can report it appropriately?

[jugglinmike]

I think the main thing is that the host needs to be able to recognize uncaught errors generally (whether from expected negative cases or unexpected failures due to implementation bugs), so defining the behavior of $ERROR in a host-specific way would be of limited use.

[bterlson]

Yeah that makes sense! I like 33c083c so I'm going to pull this in.