release-gap-package: enhance the check whether building the documentation works

[ThomasBreuer]

If the package in question provides a file makedoc.g then release-gap-package decides about success or failure, based on the printed output of GAPDoc functions; currently only warnings about "non resolved references" are taken into account.

We could do the same as what is proposed in gap-system/gap/pull/5835:
If there are #W messages except the ones about overfull boxes then report failure.

In addition to that, we can regard a nonzero exit code after processing makedoc.g as failure, no matter why this exit code was set.
(The currently available makedoc.g files do not set an exit code, but it would make sense to change the function AutoDoc to set a nonzero exit code if the function thinks that something went wrong. This way, several makedoc.g files would set an exit code without changing their contents.)

[fingolfin]

In fact we already scan the makedoc.g output for WARNING: non resolved reference and error out if this is detected.

And I think if makedoc.g exits GAP with an exit code, then it will automatically cause release-gap-package to fail, but I didn't test it.