Fix unknown collection error name (see #224)

Author: acdha

pysolr.py

@@ -1301,7 +1301,7 @@ def getHosts(self, collname, only_leader=False, seen_aliases=None):
 
         hosts = []
         if collname not in self.collections:
-            raise SolrError("Unknown collection: %s", collname)
+            raise SolrError("Unknown collection: %s" % collname)
         collection = self.collections[collname]
         shards = collection[ZooKeeper.SHARDS]
         for shardname in shards.keys():