Fix usage of checkRequirement method (#16718)

### What does it do?
Fix wrong check of checkRequirement method

### Why is it needed?
checkRequirement result contains a string or true. So the check will
never have a false result and the error is not sent.

### Related issue(s)/PR(s)
None known.

---------

Co-authored-by: Jim Graham <[email protected]>

Author: Jako

Diff for: core/src/Revolution/Rest/modRestController.php

@@ -668,8 +668,9 @@ public function post()
         $properties = $this->getProperties();
 
         if (!empty($this->postRequiredFields)) {
-            if (!$this->checkRequiredFields($this->postRequiredFields)) {
-                return $this->failure($this->modx->lexicon('error'));
+            $result = $this->checkRequiredFields($this->postRequiredFields);
+            if ($result !== true) {
+                return $this->failure($result);
             }
         }
 
@@ -744,8 +745,9 @@ public function put()
         }
 
         if (!empty($this->putRequiredFields)) {
-            if (!$this->checkRequiredFields($this->putRequiredFields)) {
-                return $this->failure();
+            $result = $this->checkRequiredFields($this->putRequiredFields);
+            if ($result !== true) {
+                return $this->failure($result);
             }
         }
 
@@ -819,8 +821,9 @@ public function delete()
         }
 
         if (!empty($this->deleteRequiredFields)) {
-            if (!$this->checkRequiredFields($this->deleteRequiredFields)) {
-                return $this->failure();
+            $result = $this->checkRequiredFields($this->deleteRequiredFields);
+            if ($result !== true) {
+                return $this->failure($result);
             }
         }