BaseTools/PatchCheck.py: Let EmailAddressCheck describe email checked

We are checking different emails from the signature list. We are
going to check more. To be able to differency, add a description
field, so the error reported is clearer.

Cc: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Reviewed-by: Bob Feng <bob.c.feng@intel.com>
Signed-off-by: Philippe Mathieu-Daude <philmd@redhat.com>