Differentiate between wrong password and communication error

[loongyh]

• Describe the scope of your change--i.e. what the change does and what parts
  of the code were modified. This will help us understand any risks of integrating
  the code.

Allow public access to checkPassword() to return one of FINGERPRINT_OK, FINGERPRINT_PACKETRECIEVEERR, FINGERPRINT_PASSFAIL. Useful to determine if failure is due to wrong password or communication error.

Also fix comment for FINGERPRINT_PASSFAIL (0x13) to reflect the actual meaning as described in the datasheet:

• Describe any known limitations with your change. For example if the change
  doesn't apply to a supported platform of the library please mention it.

Does not affect existing functionality.