Minor refactoring

thefuck/exceptions.py

@@ -1,2 +1,6 @@
 class EmptyCommand(Exception):
-    """Raises when empty command passed to `thefuck`."""
+    """Raised when empty command passed to `thefuck`."""
+
+
+class NoRuleMatched(Exception):
+    """Raised when no rule matched for some command."""