Linux/udisks2/image customization: fix issue with mounting NVMe drives

When using image customization with an NVMe drive under RPI OS on a CM4,
the system will prompt non-root users for password when mounting the FAT file system.
(Passwordless mounting does is available for removable storage,
but the NVMe drive qualifies as internal storage instead).

- The mount command we send to udisks2 over dbus2 may timeout
before the user completed the password prompt.
Raise timeout to counter this.

- Print mount error details to console
- On mount error report error mounting in GUI instead of
  "/config.txt does not exist" (which happened when mountDevice()
  returned empty string)

downloadthread.cpp

@@ -916,7 +916,9 @@ bool DownloadThread::_customizeImage()
             /* Not running as root, try to outsource mounting to udisks2 */
     #ifndef QT_NO_DBUS
             UDisks2Api udisks2;
-            mountpoints.push_back(udisks2.mountDevice(fatpartition).toStdString());
+            QString mp = udisks2.mountDevice(fatpartition);
+            if (!mp.isEmpty())
+                mountpoints.push_back(mp.toStdString());
     #endif
         }
         else