String representation of stbt.MatchTimeout & MotionTimeout

This is useful so that you can print `str(e)` for a MatchTimeout `e` in
your own exception handler (it's often useful to catch these exceptions
yourself instead of letting them bubble up to `stbt run`).
Tweak wording of stbt.MatchTimeout and MotionTimeout

To be completely unambiguous, it says that we didn't find the
match/motion within the entire `timeout_secs` time; not that we started
looking after `timeout_secs` and then didn't find it.
stbt run: Print exception name & message, not just stack trace

The output for exceptions caught by `stbt run` (i.e. *all* exceptions,
since commit 4de977c4) is now identical to what the python interpreter
prints if you don't catch the exception.

I'm not terribly sure that printing some stuff to stderr and some to
stdout is very useful here; but I've kept the extra "FAIL" line printed
to stdout in case somebody's script depends on it. I changed `print` to
`stdout.write` just to be more explicit about it. Note that
`traceback.print_tb` prints to stderr.
irnetbox.py: Cleanup: Remove unused `MESSAGE_NAMES` dict

Unused since commit da0c2ead.
stbt.py: put sink_bin back to PLAYING state after restarting source_bin

to avoid a bug where Blackmagic Intensity Pro video capture cards
stop delivering timestamp information after the pipeline is restarted.

Since b6135eca the source pipeline is restarted whenever video signal is
lost to resolve the HD PVR bug that the device stops delivering video
when it loses input signal momentarily. Unfortunately, the recovery
procedure introduced a problem that the Blackmagic card stops delivering
the video timestamp in each video frame after the source bin is
restarted, even though it does continue to deliver video frames, as it
can be seen in this log snippet:

"Weak match found: MatchResult(timestamp=0L, match=False,
position=Position(x=247L, y=382L), first_pass_result=0.6553829610347748)"

Because stb-tester's timeouts are based on the stream timestamp,
not on clock time, "stbt run" hangs forever.

After some experimentation it has turned out that timestamps are lost
because sink_bin is set to STATE_READY but not set explicitly back to
STATE_PLAYING after source_bin is restarted.

We've tested powering on/off the system-under-test and the pipeline can
succesfully recover using both Blackmagic and HD PVR devices.

Signed-off-by: Mate Szendro <mate.szendro@youview.com>