Improve several Diffusion UI error states

Summary:
Give users better errors and UI:

• For subpath SVN repositories, default the path to the subdirectory, not to

"/". This makes the home screen useful and things generally less confusing.

• For unparsed commits, show a more descriptive error message without the

"blah blah" silliness.

• For paths outside of the subpath parse tree, short circuit into an

appropriate error message.

• For foreign SVN stub commits (see D892), show an explicit message.

Test Plan: Looked at unparsed commits, subpath repositories, foreign stub
commits, and paths outside of the subpath parse tree. Received sensible error
messages.

Reviewers: jungejason, nh, tuomaspelkonen, aran

Reviewed By: jungejason

CC: aran, jungejason

Differential Revision: 894