Sending a garbage value to check_bewit should raise HawkFail

[kumar303]

If you do something like this:

from mohawk.bewit import check_bewit
check_bewit(
    'https://site.org/protected-image.png?bewit=just-a-random-string', 
    credential_lookup=...
)

... you'll get something like:

    Traceback (most recent call last):
      File "/usr/local/Cellar/python@2/2.7.15_1/Frameworks/Python.framework/Versions/2.7/lib/python2.7/doctest.py", line 1315, in __run
        compileflags, 1) in test.globs
      File "<doctest usage[14]>", line 1, in <module>
        check_bewit('{url}?bewit={bewit}'.format(url=url, bewit='INVALID'), credential_lookup=lookup_credentials)
      File "/Users/kumar/dev/mohawk/mohawk/bewit.py", line 124, in check_bewit
        bewit = parse_bewit(raw_bewit)
      File "/Users/kumar/dev/mohawk/mohawk/bewit.py", line 77, in parse_bewit
        decoded_bewit = b64decode(bewit).decode('ascii')
      File "/usr/local/Cellar/python@2/2.7.15_1/Frameworks/Python.framework/Versions/2.7/lib/python2.7/base64.py", line 78, in b64decode
        raise TypeError(msg)
    TypeError: Incorrect padding

This is understandable but it all exceptions raised in check_bewit should derive from HawkFail so they can be caught more easily.