Add ability to reset $error

[renjinsk]

In case the worker is run as a CLI command and an exception is thrown, the $error property storing the message can never be reset leading to false reports in case in case you use, say LoggerInterface::error() to report it.

Add a method to reset the $error property, like a setter or make the property's visibility to protected.
At the moment, the only way to "reset" it, is to use reflection and set the property to accessible.