Check for result of openssl_pkcs12_read

[nkajic]

• Please check for result when executing openssl_pkcs12_read
  https://github.com/trive-digital/fiskalapi/blob/master/lib/Client.php#L396
  openssl_pkcs12_read returns false if there is problem with certificate / private key
  http://php.net/manual/en/function.openssl-pkcs12-read.php

• openssl_error_string can provide message for exception

• please define $certificate = []; before using it in openssl_pkcs12_read