Raise KeyError instead of a string exception.

Fixes debian bug #585225:
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=585225

gitstats

@@ -1112,7 +1112,7 @@ class GitStats:
 			if o == '-c':
 				key, value = v.split('=', 1)
 				if key not in conf:
-					raise 'Error: no such key "%s" in config' % key
+					raise KeyError('no such key "%s" in config' % key)
 				if isinstance(conf[key], int):
 					conf[key] = int(value)
 				else: