Update the warning text when building vocab on a trained w2v model (p…

…iskvorky#1190)

Updated the error message for the case when build_vocab() is triggered more than once without update parameter.

model = gensim.models.Word2Vec(sentences,min_count=3,trim_rule=my_rule)
model.build_vocab(sentences) #throws error
model.build_vocab(sentences, update = True) #works as expected

gensim/models/word2vec.py

@@ -727,7 +727,7 @@ def finalize_vocab(self, update=False):
     def sort_vocab(self):
         """Sort the vocabulary so the most frequent words have the lowest indexes."""
         if len(self.wv.syn0):
-            raise RuntimeError("must sort before initializing vectors/weights")
+            raise RuntimeError("cannot sort vocabulary after model weights already initialized.")
         self.wv.index2word.sort(key=lambda word: self.wv.vocab[word].count, reverse=True)
         for i, word in enumerate(self.wv.index2word):
             self.wv.vocab[word].index = i