Improve error output on upload

[trainman419]

Two patches:

1. Improve the representations of the ChangesFile object so that it tells us which file it was constructed from
2. Display which packages were found that were invalid.

[tfoote]

What's the difference between extraneous and invalid packages?

[trainman419]

extraneous is a set of ChangesFile objects that indicate which changes files are problematic. invalid_changes is new; it's a list of the binary package names from within those changes files that were declared be be invalid.

The result here is that the error output is now something like [ ['python-foo-bar'] ] instead of [ ChangesFile(...) ]