Add ability to return a warning instead of error when checks fail

[sabina-talipova]

Description

CacheHeadersCheck::check() method checks that correct caching headers are present. Method should return EnvironmentCheck::WARNING if validation result messageType is ValidationResult::TYPE_WARNING. Necessary to add additional check for message type.

See:

silverstripe-environmentcheck/src/Checks/CacheHeadersCheck.php

Lines 92 to 96 in af5eb42

	// @todo Ability to return a warning
	return [
	EnvironmentCheck::ERROR,
	$this->getMessage(),
	];