Fix basedir error for the validate diff resource.
Fix basedir error for the validate diff resource.

When trying to post against a repository that requires a base directory and not
providing it, the resulting error was being split into a list of characters.
This was happening because we were returning a string as the error instead of a
list, so iterating over it was turning it into a list.

Testing done:
Verified that the error was presented as a readable string instead of a list of
individual characters.

Reviewed at https://reviews.reviewboard.org/r/7127/

davidt/reviewboard,davidt/reviewboard,reviewboard/reviewboard,chipx86/reviewboard,chipx86/reviewboard,bkochendorfer/reviewboard,sgallagher/reviewboard,KnowNo/reviewboard,brennie/reviewboard,sgallagher/reviewboard,reviewboard/reviewboard,reviewboard/reviewboard,KnowNo/reviewboard,chipx86/reviewboard,sgallagher/reviewboard,bkochendorfer/reviewboard,custode/reviewboard,bkochendorfer/reviewboard,sgallagher/reviewboard,beol/reviewboard,reviewboard/reviewboard,custode/reviewboard,beol/reviewboard,davidt/reviewboard,chipx86/reviewboard,KnowNo/reviewboard,brennie/reviewboard,KnowNo/reviewboard,beol/reviewboard,brennie/reviewboard,bkochendorfer/reviewboard,davidt/reviewboard,custode/reviewboard,brennie/reviewboard,beol/reviewboard,custode/reviewboard