fix #333. Readable error msgs and don't create user if pass is invalid

Currently we create a user with no password if the password provided was
not more than 4 chars and less than 56 chars. We also return an error
msg that reveals the internal of our crypto algorithm and isn't readable
by the end user.
Merge pull request #333 from influxdb/fix-333-crypto-bug

"Failed to create user: crypto/blowfish: invalid key size 1"