add: introduce error codes

[javereec]

Just an idea I had to improve how errors can be extended with codes allowing for better handling by clients.

I think it also fixes some other issues in the specs

[vijayshiyani]

Good idea, I changed how error messages structure.
TO DO:

• Fix error messages
• Fix tests

[linasi]

Looks good. I would suggest to keep it a bit more simple.

• extra_info related functionality should be dropped
• remove getResponse method, error message is available via error.message that comes form the base class.
• remove equals method

I am not so sure about having code and errorType attributes together. They seem to overlap a little. Are there any rules on how they are defined? Wouldn't just having one be enough? Also, shouldn't code be a unique value?