Output better error messages on certain edebug spec errors

Output better error messages on certain edebug spec errors

This fixes bug#65620.  The error thrown up to now was "Void
function edebug-after".  This has been replaced by "Invalid
call to `edebug-before'.  Is the edebug spec for `foo'
correct?".

* lisp/emacs-lisp/edebug.el (edebug-b/a-error): New function
which throws the new friendlier error message.
(edebug-before, edebug-after): Replace the defalias's to nil
with actual functions which just call edebug-b/a-error.

* doc/lispref/edebug.texi (Specification List): In the entry
for `sexp', warn against mistakenly using `form' for an