Correct naming error in exception-showing

Late edit to #437 missed a rename of `num` to `exc_num` - code was
showing external interrupts successfully, but trapped if faced with an
internal exception.

pyocd/coresight/cortex_m.py

@@ -977,7 +977,7 @@ def exception_number_to_name(self, exc_num, name_thread=False):
             if exc_num == 0 and not name_thread:
                 return None
             else:
-                return self.CORE_EXCEPTION[num]
+                return self.CORE_EXCEPTION[exc_num]
         else:
             irq_num = exc_num - len(self.CORE_EXCEPTION)
             name = None