"invalid mnemonic" message could be more helpful

[abliss]

If one word is misspelled (i.e. does not appear on the wordlist), the message should tell me which one. If the string doesn't parse because I accidentally put two spaces instead of one somewhere, it should tell me where. (These are perhaps better made as requests against bip39.js?)

Also, since the mnemonicToSeed is to be used instead of mnemonicToEntropy, there should (as recommended by the bip39 readme) be an option to bypass the check and proceed anyway (in case an alternate or obsolete wordlist was used).