Add return/report/warnings_as_errors compile options [JIRA: RIAK-2281]

[ghost]

It would be highly useful to add the same return/return_warnings/return_errors/report/report_warnings/report_errors/warnings_as_errors options as available in compile:file and leex:file for erlang_protobuffs' own errors/warnings.

[lordnull]

This is already done:

protobuffs_compile:scan_file("messages.proto", [{compile_flags, [warnings_as_errors]}]).

These are passed to the call to compile:file/2 with 'report' prepended. What needs to happen is actual documentation of the options.

[ghost]

@lordnull, does that mean erlang_protobuffs doesn't emit warnings of its own? Options passed to compile:file of course support warnings_as_errors, but the ticket is about warnings emitted by this compiler.

[lukebakken]

A PR with this feature and tests would be appreciated. If one is submitted, please reference this issue and re-open it. Thank you.