Raise ValueError rather than logging in parse.padBase64().
Only return the re-padded base64 string if no errors were raised.

 * CHANGE parse.padBase64() to only return a value if no exceptions were
   raised during parsing.
Move base64 re-padding logic into separate function.

This logic is not specific to parsing networkstatus descriptors, so for now
I've placed it in parse.parseUnpaddedBase64() ― though it could live somewhere
else later.
Change parse.networkstatus unittest values to be clearer when decoded.

Store the pre-base64-encoded value so that the checks are clearer if they
fail. Also, remove a str() around a couple `nick` values, they do not make a
difference now that the proper __future__ imports are in place.
Add three new unittests for parseRLine() base64 encoding issues.