Do not throw an exception when the username does not exist

refs #6457

library/Icinga/Authentication/Backend/LdapUserBackend.php

@@ -94,16 +94,18 @@ class LdapUserBackend extends UserBackend
      * @param   User    $user
      * @param   string  $password
      *
-     * @return  bool|null
-     * @throws  AuthenticationException
+     * @return  bool    True when the authentication was successful, false when the username or password was invalid
+     * @throws  AuthenticationException When an error occurred during authentication
      */
     public function authenticate(User $user, $password)
     {
         try {
-            return $this->conn->testCredentials(
-                $this->conn->fetchDN($this->createQuery($user->getUsername())),
-                $password
-            );
+            $userDn = $this->conn->fetchDN($this->createQuery($user->getUsername()));
+            if (!$userDn) {
+                // User does not exist
+                return false;
+            }
+            return $this->conn->testCredentials($userDn, $password);
         } catch (Exception $e) {
             throw new AuthenticationException(
                 sprintf(

library/Icinga/Protocol/Ldap/Connection.php

@@ -207,16 +207,19 @@ class Connection
         return true;
     }
 
+    /**
+     * Fetch the distinguished name of the first result of the given query
+     *
+     * @param       $query
+     * @param array $fields
+     *
+     * @return bool|String   Returns the distinguished name, or false when the given query yields no results
+     */
     public function fetchDN($query, $fields = array())
     {
         $rows = $this->fetchAll($query, $fields);
         if (count($rows) !== 1) {
-            throw new \Exception(
-                sprintf(
-                    'Cannot fetch single DN for %s',
-                    $query
-                )
-            );
+            return false;
         }
         return key($rows);
     }