Fix two serious flake8 reports

flake8 points out that some code in frame_filters.py is referring to
undefined variables.

In the first hunk, I've changed the code to match what other
'complete' methods do in this file.

In the second hunk, I've simply removed the try/except -- if
get_filter_priority fails, it will raise GdbError, which is already
handled properly by gdb.