Scrooge generation uses an absolute path. However the jvm_compile._register_products method assumes paths relative to the build root, causing it to miss classes.
Added two example (java/scala) projects that depend on code generated from thrift, they no longer throw an incorrect dependency warning
Changed the error message received when specifying an invalid target in a BUILD file to actually be helpful.

Previously pants just choked with a ValueError stating that the spec was not found in the BUILD file. Now it actually lists valid specs that are contained in the build file, so that the user can recover easily without inspecting the BUILD file's contents.

See issue: https://github.com/pantsbuild/pants/issues/224