CI: Elevate warning to error if release version does not match.

[mmuetzel]

Also, avoid contraction in message.

Apparently, updating the DESCRIPTION file for a package release is an easily missed step. At least, that was happening a couple of times in the last few weeks.
Failing the CI in that case might raise awareness for that step.

I'll leave this open for at least a week or so and wait for feedback before merging.
If there is earlier agreement that this is something that we should do, we can also merge earlier.

[pr0m1th3as]

I support this change.

[mmuetzel]

@pr0m1th3as: Thanks.

I didn't look close enough what step_error actually does. I followed up with a change that actually marks the CI as "failed" if the versions do not match. (But still run the remaining steps.)

[pr0m1th3as]

I didn't look close enough what step_error actually does.

I didn't look close enough too. Sorry for that, @mmuetzel. I just thought that step_error is the error handler function.