NMEASentence/__getattr__: Graceful fail

[barbacbd]

** Return None and handle the error without raising an error. Technically
raising an error after catching another is bad as it still creates a stack
trace (causing a hard failure) and it demolishes the original stack. Here the
return None is handled and a message is logged in debug format. The other way
to resolve this on a hard failure is to reraise the origin after logging just call
raise to preserve the stack.

** Altered the test that utilized this code.

Fix for #94

[coveralls]

Coverage increased (+0.005%) to 98.348% when pulling e55386d on barbacbd:getattr_graceful_failout into a0d60b6 on Knio:master.

[coveralls]

Coverage increased (+0.005%) to 98.348% when pulling e55386d on barbacbd:getattr_graceful_failout into a0d60b6 on Knio:master.

[barbacbd]

/retest python 3.7

[barbacbd]

@Knio not sure if these tests can be rerun, but it didn't look like it. The python3.7 job is failing for a reason that doesn't appear to be related to the software. Can we rerun this to confirm?

[Knio]

I don't think this is a good idea to silently fail for fields that were never defined, and goes against the normal Python behavior of every other object.

It also doesn't solve #94, since that's accessing defined fields and not taking this code path, and will still fail on parsing None.

I think 94 should be solved manually in the DateTimeFix mixin.

Python3+ also properly supports raising an exception from within an exception handler, and linking it to the original exception and stack.