handle lookup errors when connecting

[ncopa]

Connect may result in lookup error, which was not handled. Solve this by use a default error handler which returns whatever mosquitto reports.

This prevents us from missing errors.

[karlp]

Given that we still have some of the error codes enumerated, would it perhaps make more sense to leave the others that were enumerated, and just add a default case, perhaps with a "warning, missing enums in library handling!" ?

[ncopa]

The idea was to only handle the special cases:

• success
• errno
• errors that should raise a lua error

And then just returns the error number with the mosquitto_strerror as default.

This helps us to deal with undocumented errors (like no connection) and future errors.

I dont think we need a warning for it.

One question though, I don't know we we should raise a lua error by default or return the error number.

[ncopa]

Given that we still have some of the error codes enumerated, would it perhaps make more sense to leave the others that were enumerated, and just add a default case, perhaps with a "warning, missing enums in library handling!" ?

Rebased it to do this, but I have no idea how to do the warning.

@karlp can you please have another look?