Leverage Sourcemaps for Closure Compiler Errors and Warnings

[kristoferbaxter]

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, 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.