Add a fail on error mode

Currently [Elemental2](https://github.com/google/elemental2) builds can produce warnings that do not halt the build. As a result it is possible for to introduce more problems in code generated from this tool without realizing it. For example a a recent commit introduced the error

```
Mar 26, 2019 8:03:44 PM com.google.javascript.jscomp.LoggerErrorManager println
WARNING: external/com_google_closure_compiler/externs/browser/url.js:162: WARNING - Bad type annotation. Unknown type MediaSource
* @param {!File|!Blob|!MediaSource|!MediaStream} obj
```

A possible solution is to update the generator tool so that it will fail if warnings that are not part of a known problems list will cause the build to fail. A possibly simpler alternative is to just introduce a fail on warning flag and fix the problems in Elemental2.