This moves the error messages for misbehavior (when available) into the line that reports the misbehavior, as well as moves the logging to the `net` category.

This is a continuation of #11583 and avoids serious-looking errors due to misbehaving peers. As it is impossible to correlate the `peer=X` numbers to specific incoming connections now without enabling the `net` category, it doesn't really help to see these messages by default.

To do this, Misbehaving() gains an optional `message` argument.

E.g. change:

    2018-01-18 16:02:27 Misbehaving: x.x.x.x:62174 peer=164603 (80 -> 100) BAN THRESHOLD EXCEEDED
    2018-01-18 16:02:27 ERROR: non-continuous headers sequence

to

    2018-01-18 16:02:27 Misbehaving: x.x.x.x:62174 peer=164603 (80 -> 100) BAN THRESHOLD EXCEEDED: non-continuous headers sequence

When there is a category for "important" net messages (see #12219 ), we should move it there.

make check
test_runner.py