Fix "error on writing to disk" that sometimes appears after "cancel"

- Do not generate "error on writing to disk" error if it is the
  result of the write being cancelled.
- Mac: QProcess::execute(cmdline) -> QProcess::execute(cmd, args)
  to avoid depreceation warning.

downloadthread.cpp

@@ -196,7 +196,8 @@ bool DownloadThread::_openAndPrepareDevice()
     } else if (authopenresult == _file.authOpenError) {
         QString msg = tr("Error running authopen to gain access to disk device '%1'").arg(QString(_filename));
         msg += "<br>"+tr("Please verify if 'Raspberry Pi Imager' is allowed access to 'removable volumes' in privacy settings (under 'files and folders' or alternatively give it 'full disk access').");
-        QProcess::execute("open x-apple.systempreferences:com.apple.preference.security?Privacy_RemovableVolume");
+        QStringList args("x-apple.systempreferences:com.apple.preference.security?Privacy_RemovableVolume");
+        QProcess::execute("open", args);
         emit error(msg);
         return false;
     }
@@ -439,6 +440,7 @@ void DownloadThread::run()
             }
             else
 #endif
+            if (!_cancelled)
                 _onDownloadError(tr("Error writing file to disk"));
             break;
         case CURLE_ABORTED_BY_CALLBACK: