Add assert_list to ensure an argument is a list

Testing Done:
Ran unit tests locally

Reviewed at https://rbcommons.com/s/twitter/r/811/
Updates error message when referencing the wrong path to a BUILD file.

Before, it would just spit out the incorrect BUILD file path.:

  Exception message: BUILD file does not exist at: /Users/zundel/Src/Pants/src/java/com/pants/examples/annotation/exampleTYPO

Now it will read as follows:

  Exception message: BUILD file does not exist at: /Users/zundel/Src/Pants/src/java/com/pants/examples/annotation/exampleTYPO
    referenced from src/java/com/pants/examples/annotation/processor:processor
    referenced from src/java/com/pants/examples/annotation/main:main

Testing Done:
Updated unit tests, ci passed at https://travis-ci.org/pantsbuild/pants/jobs/31812266

Bugs closed: 442

Reviewed at https://rbcommons.com/s/twitter/r/841/
Miscellaneous fixes to protobuf codegen.

Sorted protobuf paths and resources, and handled collisions deterministically. This process generates
copious warning/error log output where appropriate, as protobuf collisions are something the user should
definitely be aware of.

The imports property of java_protobuf_library was changed slightly to not cache its list of jars if it doesn't
manage to resolve one of its targets. This solves a problem where it incorrectly saves an empty list if it's
poked before the build graph loads.

Modified the log outputs of ivy imports to be more accurate/helpful.

Reduced occurance of extraneous/incorrect 'unconsumed gen targets' warning (it still happens, not as much).