Merge #11319: [qa] Fix error introduced into p2p-segwit.py, and prevent future simila...

Merge #11319: [qa] Fix error introduced into p2p-segwit.py, and prevent future similar errors

f97ab35fa qa: Fix bug introduced in p2p-segwit.py (Suhas Daftuar)
a7820422e qa: Treat mininode p2p exceptions as fatal (Suhas Daftuar)

Pull request description:

  #11121 inadvertently broke the constructor for the `TestNode()` object in `p2p-segwit.py`, silently breaking at least one of the tests.

  Although the python code was raising exceptions due to a `TestNode()` object not existing (or having the right type), mininode was masking these from anyone running the test through the test_runner (like travis), because it catches all exceptions during message delivery and just prints a log message and continues.  Such "graceful" handling of errors is almost certainly something we don't want in our test suite, so the first commit here attempts to prevent that type of failure from ever being masked.

  The second commit fixes the particular bug in `p2p-segwit.py`.

Tree-SHA512: b6646e3cb1e05c35c28e8899c44104bf2e2d0384643ca87042ab7f6ec0960d89f5bf25a7b95bab6e32d401c20a6018226160500f6ddceb923e81ffb04adb4f2f