Improve state script version error messages

When the state script "version" file is missing or wrongly formatted, it is hard to figure out why the state scripts are not run. See the community list threads State scripts supported versions and statescript: supported versions does not match.

Acceptance criteria

• The version is correctly parsed if it is followed by a newline
• There is an error message if state scripts are used and the /etc/mender/scripts/version file is missing, explaining that it must be in place (and ideally what the latest supported version of this Mender client is)
• There is a separate error message if the version file exists but is parsed incorrectly, explaining that it needs to contain a single number (and ideally what the latest supported version of this Mender client is)