Merge pull request #8 from freesinger/bug-fix

[bug] fix raise string error

Author: freesinger

cluster.py

@@ -51,7 +51,7 @@ def classify(self, taginfo, srt_dens, min_num, maxid):
                 if taginfo[i] == -1:
                     taginfo[i] = taginfo[min_num[i]]
             except KeyError:
-                raise 'Key error: key does not exist!'
+                raise KeyError('Key error: key does not exist!')
 
         return taginfo