(WIP) Add a simple check against functions containing both return; and return <obj>;

[stevelinton]

Adds a simple check as discussed. Ignores extra return added at the end of functions.

Throws up many warnings in library and packages.

warns.txt

[olexandr-konovalov]

That's quite illuminating, and the failed travis test is easily fixable.

[olexandr-konovalov]

In the first two randomly picked up cases, I've seen a redundant return; after Error -

  if not IsSemigroupCongruence(cong) then
    Error("usage: the argument should be a semigroup congruence,");
    return;
  fi;

• maybe most of these cases fall under the same pattern.

[ChrisJefferson]

I believe (I have said this before) that most cases like that should be changed to ErrorMayQuit, which ensures users can't type return in the break loop which is what this return is there for, but that would be a separate patch.

[fingolfin]

Awesome. And the very first warning actually looks like an actual error (not just a case where Error should be replaced by ErrorMayQuit. I think getting fixing all these warnings, and merging this patch, would be worthwhile.
Of course not for 4.8 (but since we already branched stable-4.8, that should be clear anyway)

[fingolfin]

I have updated and extended this in PR #1541, so closing this PR now.