protobuffers know what files they will generate for zips/jars.

Preliminary support for compressed archives as sources. Currently .zips, .jars, tars, tar.gzs, and tar.bz2s work for protobuffers. Code is fairly extensible, with generic functionality in zipped_sources such that not much code is needed in protobuf_gen (or other targets in the future).
If you don't want an ugly exception, you need to check to see if you are running off the end of the path (case where it ends with slash)