Extra error message from update_watchpoint

Extra error message from update_watchpoint

  https://sourceware.org/ml/gdb-patches/2013-10/msg00551.html

gdb/ChangeLog

	* breakpoint.c (update_watchpoint): Update error message and add
	an additional error message.

gdb/testsuite/ChangeLog

	* gdb.base/watchpoint.exp (test_no_hw_watchpoints): Add additional
	tests and update expected error message.
	(test_watch_register_location): New tests.
	(do_tests): Call test_watch_register_location.
	* gdb.base/watchpoints.exp: Update expected error message.