Fix several error handling issues with Subversion commits in Diffusion

Summary:
Ref T9513. I checked this briefly but didn't do a very thorough job of it.

• Don't try to query merges for Subversion, since it doesn't support them.
• Fix up "existsquery" to work properly (and efficiently) for both hosted and imported repositories.
• Fix up "parentsquery" to have similar behavior on invalid commits to other VCSes (throw an exception).

Test Plan:

• No more merges warning on SVN.
• Hosted SVN gets the right exists result now.
• Visiting "r23980283789287" now 404's instead of "not parsed yet".

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9513

Differential Revision: https://secure.phabricator.com/D14239