This behavior became broken again after switching away from the
macro-based command parsing. The bot would return any non !command
message as an error, which would cause it to read more messages, and
return those as errors, until finally the matrix SDK would throw up.
Command parser now more properly handles empty messages and
non-commands, but we also simply abort processing if the incoming
message doesn't start with an exclamation point.
By using the alpha1 function in complete mode, we are able to handle
arbitrary single-word commands (e.g. "!help") and proprly map the
remaining input to an empty string.