Figure a consistent way of error handling

[keneanung]

While the usual lua idion is returning nil and an error messages in case of an error, this does not always work (like in constructors).

We should definitely use this scheme in the interface (the static MSP class) and we should handle this on the Mudlet side appropriately.

Inside the library I'd like avoid the nil checks and so on. There are sme options:

• https://github.com/leafo/moonscript/wiki/Exception-handling
• https://github.com/hjelmeland/try-lua
• http://lua-users.org/wiki/FinalizedExceptions

We need to evaluate pros and cons of each option and eventually we should choose a single option to be consistent.