There's no reason we should wait 3s before retrying a failed AT command.
This can even cause issues with suspend inhibition: 3 retries of a
failed command would take 9s, while logind ignores the inhibitor after
only 5s.
Reducing the delay to 500ms should be enough and wouldn't interfere with
suspend inhibition.
Fixes#3
The response can include the command and an error number, so we want to
only check it contains ERROR, even if it's replying more than just the
'ERROR' string.
