raise exception at end of plural5.plural() if no rule matched. a rule should always match

examples/plural5.py

@@ -24,6 +24,7 @@ def plural(noun):
     for matches_rule, apply_rule in rules():
         if matches_rule(noun):
             return apply_rule(noun)
+    raise ValueError('no matching rule for {0}'.format(noun))
 
 if __name__ == '__main__':
     import sys

generators.html

@@ -304,7 +304,8 @@ rules = []
 def plural(noun):
     for matches_rule, apply_rule in rules():
         if matches_rule(noun):
-            return apply_rule(noun)</code></pre>
+            return apply_rule(noun)
+    raise ValueError('no matching rule for {0}'.format(noun))</code></pre>
 
 <p>How the heck does <em>that</em> work? Let&#8217;s look at an interactive example first.