Catch `JWSError`s in `jwt.decode()`
Catch `JWSError`s in `jwt.decode()`

So far exceptions raised in `jws.verify()` weren't caught in the above
function, which led to it raising (undocumented) exceptions from the
underlying module.

This commit transforms said exceptions. This includes cases of invalid
payload padding, error handling for which had previously been attached
to the `json.loads()` call.

mpdavis/python-jose