3195: Improve error handling in Job creation API.

* Express the default filters using high-level operators, then convert
  them to SQL filters using the same conversion logic users get.

* Raise an ArgumentError when a git version search fails.

* Render normal error pages from ArgumentErrors.
Improve API server's git rev-list calls.

This doesn't completely silence errors, but it makes them less verbose
and prevents git from trying to look up paths, which we never want.
Merge branch '3195-bugfix-job-min-script-ver'

Closes #3195.  There may be other issues here but we need more
information to debug them.