Error messages in separate modules

[sept-en]

This is a follow-up on #535 where constant error messages were introduced. The suggestion is also to separate these errors logically into appropriate modules. This will allow selecting the right error for a given scope of the code and ensuring that "foreign" errors are not used where they are not intended to be used.

Some ideas of such modues:

• evm module: e.g. errors::evm::ERR_OUT_OF_GAS;
• json module: e.g. errors:json::ERR_NOT_A_JSON_TYPE;
• etc.