clarified the usage of the error class

[4mig4]

As the subject says

[coveralls]

Coverage remained the same at 63.439% when pulling 3ff4e49 on 4mig4:master into 29d8667 on Lua-cURL:master.

[coveralls]

Coverage remained the same at 63.439% when pulling 3ff4e49 on 4mig4:master into 29d8667 on Lua-cURL:master.

[moteus]

This is not fully correct. You can use error class as Lua error for Lua>5.1.
Also, I think documentation should mentionthat for Lua 5.1 library rises string errors.