Scrypt.Enc: longer description of constant nonce
README: improve wording
Terminology fix; use "password" rather than "key" throughout

Using the word "key" about the password can be misinterpreted
as referring to the encryption key.
Scrypt.Enc: expose CryptError constructors
Scrypt.Enc: document CryptError constructors
Scrypt.Enc: improve Show instance for CryptError