Fixed error handling in mn_decode()

[snej]

Fixed two bugs that kept mn_decode from reporting any errors:

1. The error check for mn_next_word_index was inside the loop, not outside, so it never triggered.
2. There was no check of the return value of the last mn_decode_word_index call, so no decoding errors were detected.

[singpolyma]

Thanks for this! That condition being inside the loop seems like a pretty big omission in the original...