When stopping a keygrabber with a timeout manually or through the stop
key, the timer would continue and call the stop callback again some time
later.
The error message in `gears.timer:stop` is removed, since there actually
is no harm in just returning immediately. And the timer implementation
itself calls `:stop` in certain places without checking for `.started`,
which lead to a situation where the internal call to `stop` triggered
the error message.
Signed-off-by: Lucas Schwiderski <lucas@lschwiderski.de>
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