handle importXmlInternalError gracefully in RO.php

parseResponse() was throwing "Cannot access offset of type string on string" from line 1301 when response from FlexiBee came with following data:
```
["@Version" => "1.0", "success" => "false", "stats" => ["created" => "0", "updated" => "1", "deleted" => "0", "skipped" => "0", "failed" => "0"], "results" => [["errors" => [["message" => "Interní chyba aplikace.", "messageCode" => "importXmlInternalError"]]]]]
```
That made it hard to diagnose problems, because it stripped out relevant information from FlexiBee response. This commit adds fallback for this case when message is not found where expected.

Author: ovalek

src/AbraFlexi/RO.php

@@ -1298,7 +1298,14 @@ public function parseResponse($responseDecoded, $responseCode)
 
                 // no break
             case 401:
-                $msg = (\array_key_exists('message', $responseDecoded) ? $responseDecoded['message'] : $responseDecoded[key($responseDecoded)]['message']).' for '.$this->getApiURL();
+                if (\array_key_exists('message', $responseDecoded)) {
+                    $msg = $responseDecoded['message'];
+                } elseif (isset($responseDecoded[key($responseDecoded)]['message'])) {
+                    $msg = $responseDecoded[key($responseDecoded)]['message'];
+                } else {
+                    $msg = sprintf("Unexpected response data [%s]: %s", $responseCode, json_encode($responseDecoded));
+                }
+                $msg .= ' for '.$this->getApiURL();
                 $this->addStatusMessage($msg, 'error');
 
                 if ($this->throwException) {