Gerrit plugin tracebacks for non-existent user

[psss]

When an email of non-existent user is provided searching for gerrit changes results in a traceback. This should probably just raise a warning or silently ignore this particular error. Otherwise it's not possible to create team reports with only some users having account on the gerrit instance.

Unfortunately it seems the only way how to detect the problem by checking the 400 error code from the server, no further details. So ignoring this could hide other problems as well. Not sure what's the best solution here.