Ensure extended validators can create keywords which override exception messages

Hi, first of all, thanks for all your work with this package.

All in all i think it works great and really does what it should and is supposed to do.
I think that if i should mention anything that i think could be better it would probably be better error messages, now i do understand that it is really hard to generate good error messages for all use cases but something that could be very useful is the option to add custom error messages.

Here is an example.

{
    "type": "object",
    "properties": {
        "templates": {
            "type": "object",
            "properties": {
                   "path": {"type": "string", "minLength": 1, "pattern": "^(/[^/]+)+$", "message": {"pattern": "You must use a valid full path with no trailing slashes"}}},
            "required": ["path"],
        },
    },
    "required": ["templates"],
}

I've read the documentation and haven't been able to find a way to define custom messages in the schema, but maybe i have missed something?

No file or test entry point is named in the issue. Start by reading the validator and extended-validator documentation, then locate how exception messages are produced; done means an extended validator can define schema keywords that override those messages, with coverage for the example behavior.