fixup! Refactor, extract phlcon_user.response_from_att...
Refactor, phlcon_user error names to end in Error

Follow the convention of custom exception names ending in 'Error' so
it's obvious that they represent errors and would be expected to inherit
from 'Exception'.

Applied refactoring with rope.

Test Plan:
$ ./precommit.sh

Make sure we didn't leave any instances behind
$ git grep 'OneOrMoreUnknownUsernames\b\|OneOrMoreUnknownPhids\b\|UnknownUsername\b\|UnknownPhid\b'
.. no results ..
.. try the same but with 'Error' at the end of one of the names ..
.. results ..