Better pattern matching and dump of warning messages.

* Warning messages must start with source file path.
  This will reject lines containing only 'warning:'
  but not a source file path prefix.
* Escape both backslash and quotation marks in the
  dump of strings to JavaScript string literals.

Bug: 32060052
Test: run through build.log files
Change-Id: Ib064768b1ba2954f974604ea054f7a6d2ad15ae7