Improve error message when regex doesn't match. (#184)

Point the user at the source of the regex. This would have saved me time
when encountering the error in `riscof` which gave me no indication of
what the actual problem was.

Author: rtwfroody

riscv_config/schemaValidator.py

@@ -110,7 +110,8 @@ def _check_with_capture_isa_specifics(self, field, value):
             self._error(field, "Invalid width in ISA.")
 
         if not constants.isa_regex.match(value):
-            self._error(field, 'Input ISA string does not match regex')
+            self._error(field, 'Input ISA string %r does not match regex from %s' % (
+                value, constants.__file__))
         if ext_err:
             for e in ext_err_list:
                 self._error(field, e)