Change gimp CAPTCHA encoding strategy to assume HMAC is 20 bytes.

 * CHANGE relevant unittests which looked for the original ';' string
   delimiter to now split challenge string, taking the first 20 bytes as
   the HMAC, and the rest as the encrypted blob containing the HMAC
   answer.

 * CHANGE `bridgedb.captcha.GimpCaptcha.check()` to always immediately
   return False whenever any Exception occurs.

 * ADD eight new unittests for `captcha.GimpCaptcha.check()`.
Catch gimp CAPTCHA challenge decryption ValueErrors.

  * ADD unittest will raise a ValueError during decryption with the
    previous commit's code.

  * ADD exception handling to `captcha.GimpCaptcha.check()` to cover
    this case.