Generalize errors to allow better handling

[masked-rpgmaker]

Feature: Be able to have generic error objects with contextual information instead of just a string message.

Desired properties:

• Have different error descriptors for different input types;
• Possibly be able to have different error descriptors for two parsers with the same input type;
• Be as generic as possible and as composable as possible;
• Strongly typed error descriptions by default (weak typing can still be achieved with Record);