Validation: add WithMessage

[System-Glitch]

References

Issue(s): closes #250

Description

• Added validation.WithMessage() function, which overrides the language entry used to render the error message of a validator.
  • Original placeholders returned by the validator are still used to render the message.
  • Type-dependent and "element" suffixes are not added when the message is overridden.

Example usage:

func (ctrl *Controller) UpdateRequest(_ *goyave.Request) v.RuleSet {
	return v.RuleSet{
		//...
		{Path: "contents", Rules: v.List{v.WithMessage(v.String(), "validation.rules.custom"), v.Min(10)}},
	}
}

Possible drawbacks

The WithMessage syntax is not the cleanest. However, using a method instead wouldn't be possible because it would need to return itself, which wouldn't be possible with composition.

[coveralls]

Pull Request Test Coverage Report for Build 14337030354

Details

• 14 of 14 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.005%) to 97.516%

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Totals
Change from base Build 14337018348:	0.005%
Covered Lines:	6477
Relevant Lines:	6642

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💛 - Coveralls