Dear all,

when checking expressions for array sections, we need to ensure
that these use only type INTEGER.  However, it does not make sense
to generate an internal error when encountering wrong types,
but rather take the ordinary route of error recovery.

The initial fix was provided by Steve.

While working on the same PR, I found that the comments related
to the logic needed a minor adjustment, and the logic could be
cleaned up to actually match the intended comment.

Regtested on x86_64-pc-linux-gnu.  OK for mainline?

Thanks,
Harald