Allow setting an error handler

[theintz]

This changeset introduces phpiredis_set_error_handler(), which allows a user to register a PHP callable as an error handler. This handler is called when an error is encountered in the communication with the Redis server. It should correspond to the following signature: function ($errorType, $errorMessage)

$errorType is one of PHPIREDIS_ERROR_CONNECTION and PHPIREDIS_ERROR_PROTOCOL, $errorMessage is just the message as a string.

I am already opening this pull request to get some input as to whether this approach is fine for everyone. Also my C isn't the best, so please let me know if there is a mistake somewhere.

Still left to do:

• Call the error handler in phpiredis_multicommand*() as well.
• Call the error handler for connection errors as well.
• Maybe pass the returned value from the error handler back?

[theintz]

Any feedback?

[nrk]

Sorry for the lack of feedback @theintz I've been too busy in the last couple of weeks, I hope to be able to review your PR during the weekend or at the start of next week at worst. In the meanwhile, if anyone else has feedback on this PR please feel free to comment.

Thanks!

[theintz]

Replaced by #23.