Add failed authentication logic

If the user has not added their public keypair to Gerrit, gerrit-growler
will loop. This commit excepts that case and exits gracefully.

fixes #3
Use .get() on dictionary properties

Previously, KeyErrors would be raised if `username` wasn't in the
`author` dictionary of the event. This uses the .get() method so that if
`username` isn't provided in `author`, it will return None and the fail
the conditional instead of raising a KeyError.

Fixes Issue #2
Merge pull request #4 from lbragstad/bug-2

Use .get() on dictionary properties
Merge pull request #5 from lbragstad/issue-3

Add failed authentication logic