Decode exception, errno as part of errormsg, throwDecodingExceptions (bool)

1: Decode was throwing Encode exception, fixed
2: Added errno as part of error encode/decode exception message (handy in case of unknown in switch-case)
3: Added $throwDecodingExceptions (bool)

Notes:
Please consider adjusting the JSON exceptions as true since people who will use this library expect to get errors easily by default. Others probably never used PestJSON.php but instead used Pest.php directly - the ones that upgrade probably will understand to handle those exceptions - I understand backwards compatibility was a concern here?