Better error handling in review-pr

[verdurin]

--review-pr should handle errors better.

An example is that currently if you try to use --review-pr on an easyblock PR, it fails without a useful error.

It should report that it cannot make use of --pr-target-repo=easybuild-easyblocks, for example.

[migueldiascosta]

@verdurin that particular problem was fixed in #3177

Did you run into other problems with error handling in --review-pr?