Raise error if no vocab entries found
Raise error if no vocab entries found

I passed a directory as the `-vocab-file` argument, which apparently doesn't trigger an error from `fopen`. This led t `vocab_size = 0`, which caused a segfault during execution.

I've raised an error if the vocab size is still 0 after trying to read the vocab file. This seems a bit better than checking for a directory, since we still should raise if an incorrect but existing file path was passed.
stanfordnlp/GloVe,stanfordnlp/GloVe,stanfordnlp/GloVe,stanfordnlp/GloVe