Let users return Error objects from validators

src/compose-rules.js

@@ -40,9 +40,11 @@ module.exports.compose = function composeValidators(validators) {
 					} else {
 						throw new Error(`The 'errors' in a validationResult must be an array`);
 					}
-				} else if (result instanceof Error) {
-					// We could interpret returned Errors as either values or a throw substitute. Let's wait for users to file issues, so that we know what people *actually* need here.
-					throw new Error(`It is currently not allowed to return an Error object from a validator. If you have a reason to need this, please file a bug!`);
+				} else if (result instanceof Error && matchValidationError(result)) {
+					return {
+						errors: [ result ],
+						newValue: undefined
+					};
 				} else {
 					return {
 						errors: [],