Define condition method-already-on-generic-function

[robert-strandh]

In the file Code/CLOS/add-remove-method-support.lisp, in the
function add-method-default, a simple error is signaled when an
attempt is made to add a method to a generic function, but that method
is already associated with the generic function.

To avoid the simple error, in the file Code/CLOS/conditions.lisp,
define a new condition named METHOD-ALREADY-ON-GENERIC-FUNCTION,
modeled after the other conditions in that file. Do not use the
:REPORT option. Define two slots %METHOD and %GENERIC-FUNCTION with
readers METHOD and GENERIC-FUNCTION respectively.

In the file Code/CLOS/condition-reporters-english.lisp, define
a method for reporting the new condition, modeled after the other
methods in this file.

In the file Code/CLOS/packages.lisp, shadow the symbols METHOD and
GENERIC-FUNCTION.