R1:8f3b342287bc

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.