Add argument to mypy linter to always show error codes

Summary:
This is useful for determining the exact type to use for exceptions.
There's no reason to not show the error code all the time.

Test Plan:
With python 3.7, the following gives an error (with error code):

arc lint test/functional/test_framework/txtools.py

This is fixed in a followup patch.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D11654