Add error codes for generic errors

[swehner]

This simply adds the generic errors defined in the beanstalk protocol and treats them in the client library so it'll return different error codes if one of these errors is encountered.
I think it would make sense to make these error codes available in the c++ client as well, although this would need major changes in the API, as the function only return bool now.

[deepfryed]

Neat!

As for C++, we can have the last client error code stored in the Client instance. You don't need to change the return from bool, but set the code and then access it from another public api. How does that sound ?

[swehner]

Sounds good!