[RFC] Ability to add custom error message for Validator

[iTaybb]

Is your feature request related to a problem? Please describe.
Currently, when using a Validator with a custom condition, the error message is not indicative enough.

For example:
Validator("ipify_api_list", condition=lambda x: len(x) > 5)
would result in:
dynaconf.validator.ValidationError: ipify_api_list invalid for <lambda>(['api.ipify.com']) in env main

Describe the solution you'd like
Add a new, "error_msg" option for Validator that will be shown as the error message

Additional context
I'll be glad to make a PR for this myself.

[rochacbruno]

The Validator class takes an messages attribute

    custom_msg = "You cannot set {name} to {value} in env {env}"
    settings.validators.register(
        Validator("VERSION", ne=1, messages={"operations": custom_msg}),
    )

I think it is missing in the docs

dynaconf/dynaconf/validator.py

Lines 88 to 99 in 0439bf8

	default_messages = MappingProxyType(
	{
	"must_exist_true": "{name} is required in env {env}",
	"must_exist_false": "{name} cannot exists in env {env}",
	"condition": "{name} invalid for {function}({value}) in env {env}",
	"operations": (
	"{name} must {operation} {op_value} "
	"but it is {value} in env {env}"
	),
	"combined": "combined validators failed {errors}",
	}
	)

[iTaybb]

This is true (however the key of the messages should be condition and not operations.
So this issue is about documenting it better :)

[rochacbruno]

added to the docs