Leverage Sourcemaps for Closure Compiler Errors and Warnings

## Is your feature request related to a problem? Please describe.

What would be really nice is if the plugin surfaced the warnings and also transformed the line/col numbers back to the original source files via the source maps. I do [something similar here](https://github.com/googleanalytics/autotrack/blob/master/bin/build.js#L83-L92), and it's super helpful when debugging.

## Describe the solution you'd like

Leverage sourcemaps for warnings and errors, so the input source is shown instead of the post rollup source in messages.

## Additional context

`closure-compiler-npm` does not surface this information in the same way for the `js` versus `native`|`java` versions of closure compiler. This task can either address `native`|`java` and omit `js` for a later stage, or address all three at the same time.

This is the second part of issue #42.