[mainloop-] for bindkey cmd, catch fail() from execCommand

[midichef]

This PR is for the situation when bindkey names a command that does not exist, like from this .visidatarc line: Sheet.bindkey('2', 'not-a-command')
When the user presses 2, visidata immediately exits with status -1, with no other output.

That's because fail() in execCommand() raises ExpectedException which bubbles up past run() and main_vd() to vd_cli():

visidata/visidata/basesheet.py

Lines 203 to 204 in d499914

	if not cmd:
	vd.fail('no command for %s' % (longname or keystrokes))

visidata/visidata/main.py

Lines 386 to 394 in d499914

	def vd_cli():
	rc = -1
	try:
	rc = main_vd()
	except BrokenPipeError:
	os.dup2(os.open(os.devnull, os.O_WRONLY), sys.stdout.fileno()) # handle broken pipe gracefully
	except visidata.ExpectedException as e:
	if vd.options.debug:
	raise

This PR prevents the abrupt exit and shows the fail message: no command for not-a-command

[midichef]

The commit message records why I catch Exception instead of the more specific ExpectedException. I tried for about 20 minutes to import ExpectedException in mainloop.py, but I could not figure out a way to untangle the resulting circular imports. Using Exception with exceptionCaught gets the job done, and matches the error handling elsewhere in the same function:

visidata/visidata/mainloop.py

Lines 239 to 243 in d499914

	try:
	if vd.replayOne(cmd):
	vd.replay_cancel()
	except Exception as e:
	vd.exceptionCaught(e)