Improve error handling in webapp /keys

While investigating some problems with key loading, it was very
difficult to discern what exactly went wrong while requesting keys. One
important note is that if a project does not exist, the source drivers
just add it, which leads to 500 crashes later because those empty
projects don't have the public_key attribute.

Change-Id: Ic2eb0372620798dd7ca63e9e8205c39bd4629a92
1 file changed