Improve error message for invalid patterns (#4556)

This error message felt really confusing to me and even while reading
the documentation, it was unclear how I could get my entries to be
stored in a subdirectory. I added this little hint to the error message
so that others may not run into the same issue.

Co-authored-by: Lukas Taegert-Atkinson <lukastaegert@users.noreply.github.com>

src/utils/renderNamePattern.ts

@@ -10,7 +10,7 @@ export function renderNamePattern(
 	if (isPathFragment(pattern))
 		return error(
 			errFailedValidation(
-				`Invalid pattern "${pattern}" for "${patternName}", patterns can be neither absolute nor relative paths.`
+				`Invalid pattern "${pattern}" for "${patternName}", patterns can be neither absolute nor relative paths. If you want your files to be stored in a subdirectory, write its name without a leading slash like this: subdirectory/pattern.`
 			)
 		);
 	return pattern.replace(/\[(\w+)\]/g, (_match, type) => {

test/function/samples/invalid-pattern/_config.js

@@ -6,6 +6,6 @@ module.exports = {
 	generateError: {
 		code: 'VALIDATION_ERROR',
 		message:
-			'Invalid pattern "../main.js" for "output.entryFileNames", patterns can be neither absolute nor relative paths.'
+			'Invalid pattern "../main.js" for "output.entryFileNames", patterns can be neither absolute nor relative paths. If you want your files to be stored in a subdirectory, write its name without a leading slash like this: subdirectory/pattern.'
 	}
 };