Simplify and fix arg length checks

Many error messages stated the number of accepted parameters for
various functions incorrectly. This is fixed, and at the same time
the arg length checks themselves are simplified.
Simplify removing last element of a list

The function Remove() does exactly that, by unbinding the last element
of a list (and returning it). This is also more efficient than
creating a sublist.
Replace arg length error by a single generic error message

The name of the method in which the error occurred is redundant, as
the backtrace GAP provides already contains it. And the number of
expected arguments is (a) easy to determine from the surrounding
code, and (b) is annoying to maintain.