When a file isn't found, it's highly unlikely that seeing a traceback

from Review Board will help. This change removes that from the error

message. While I was doing this, I made a couple cosmetic updates to the

error itself:

Instead of prefixing the revision with a single r character (which

  doesn't look so good for Git or other SHA-based revisions), we

  explicitly say "revision X".
Instead of suggesting that the error could be a bug in Review Board,

  I've simplified it to just saying it could be an outage or a diff

  problem.

Manually created a situation where a file wasn't found (by changing the

  source revision for a FileDiff in the db) and looked at the resulting

  error.
Ran unit tests.