Constraints error messages customization

[wyfo]

Currently, validation errors resulting of JSON schema constraints doesn't include the invalid data. This is a design choice, as it's more secure. However, not everybody does care about this level of security, and having data directly in the error message could be convenient.

By the way, even without these security concerns, error message customization is nice to have, as long as it's kept simple.

[wyfo]

Will be resolved as a side effect of #235.

Error messages can be customized using settings.errors attributes. Message are formatted using str.format according to parameters that will be described in the documentation.