dev/core#2823 Move validation into validation function

As the code comments suggest the handling of a module being unrecognised should
be handled in the validate not the enable function

I find the moduleIndex pretty nasty - all known modules
are indexed into either the TRUE key or the FALSE
key depending on whether they are enabled or not.

I'm focussing on adding getters to access that info for now
& will later change the structure when the code is not interacting
directly with the property

CRM/Core/ManagedEntities.php

@@ -142,15 +142,9 @@ public function reconcileEnabledModules() {
     // index by moduleName,name
     $decls = $this->createDeclarationIndex($this->moduleIndex, $this->getDeclarations());
     foreach ($decls as $moduleName => $todos) {
-      if (isset($this->moduleIndex[TRUE][$moduleName])) {
+      if ($this->moduleIsEnabled($moduleName)) {
         $this->reconcileEnabledModule($this->moduleIndex[TRUE][$moduleName], $todos);
       }
-      elseif (isset($this->moduleIndex[FALSE][$moduleName])) {
-        // do nothing -- module should get swept up later
-      }
-      else {
-        throw new Exception("Entity declaration references invalid or inactive module name [$moduleName]");
-      }
     }
   }
 
@@ -443,18 +437,53 @@ protected function createDeclarationIndex($moduleIndex, $declarations) {
    *   string on error, or FALSE
    */
   protected function validate($declarations) {
-    foreach ($declarations as $declare) {
+    foreach ($declarations as $module => $declare) {
       foreach (['name', 'module', 'entity', 'params'] as $key) {
         if (empty($declare[$key])) {
           $str = print_r($declare, TRUE);
-          return ("Managed Entity is missing field \"$key\": $str");
+          return ts('Managed Entity (%1) is missing field "%2": %3', [$module, $key, $str]);
         }
       }
-      // FIXME: validate that each 'module' is known
+      if ($this->moduleIsRecognised($declare['module'])) {
+        return ts('Entity declaration references invalid or inactive module name [%1]', [$declare['module']]);
+      }
     }
     return FALSE;
   }
 
+  /**
+   * Is the module recognised (as an enabled or disabled extension in the system).
+   *
+   * @param string $module
+   *
+   * @return bool
+   */
+  protected function moduleIsRecognised(string $module): bool {
+    return $this->moduleIsDisabled($module) || $this->moduleIsEnabled($module);
+  }
+
+  /**
+   * Is the module enabled.
+   *
+   * @param string $module
+   *
+   * @return bool
+   */
+  protected function moduleIsEnabled(string $module): bool {
+    return isset($this->moduleIndex[TRUE][$module]);
+  }
+
+  /**
+   * Is the module disabled.
+   *
+   * @param string $module
+   *
+   * @return bool
+   */
+  protected function moduleIsDisabled(string $module): bool {
+    return isset($this->moduleIndex[FALSE][$module]);
+  }
+
   /**
    * @param array $declarations
    *