fix: allow public errors to thread through on response (#10419)

### What?
When using `throw new APIResponse("Custom error message", 500, null,
true)` the error message is being replaced with the standard "Something
went wrong" message.

### Why?
We are not checking if the 4th argument (`isPublic`) is false before
masquerading the error message.

### How?
Adds a check for `!err.isPublic` before adjusting the outgoing message.

packages/payload/src/utilities/routeError.ts

@@ -55,7 +55,7 @@ export const routeError = async ({
 
   // Internal server errors can contain anything, including potentially sensitive data.
   // Therefore, error details will be hidden from the response unless `config.debug` is `true`
-  if (!config.debug && status === httpStatus.INTERNAL_SERVER_ERROR) {
+  if (!config.debug && !err.isPublic && status === httpStatus.INTERNAL_SERVER_ERROR) {
     response = formatErrors(new APIError('Something went wrong.'))
   }