Improve type-error messages for mismatches between header and val type

[shym]

Distinguish between parameters and return values Handle the special case of too many parameters Fix a typo in the mismatch error message

[shym]

I confess myself lazy there... I rewrote the function to test, in that particular case, whether we are testing the returned values for a function returning a tuple and, if so, expand the explanation. It might now err on the side of verbosity. I’ve added a test for that particular case.

[n-osborne]

Nice, thanks you!