generatePhetioMacroAPI should be graceful to individual repos failing.

[zepumph]

While working on phetsims/aqua#200, I found it very annoying that only a couple of repos were broken, but then whole regeneration failed. It would be better if it just told us the errors for individual repos, and returned null in those cases. The main purpose of this class is to write API files to the code base, so if only one asserts out, it just say that instead of failing to write out any.

[zepumph]

Done. This feels a lot better. Before you just got assertion messages printed out during the progress bar. Now they are all at the end, and will throw an error unless you ask for grace (which grunt generate-phet-io-api does).