Don't stop importing TMXs on first error

This is (temporarily) uglier, but more functional, UI. Don't terminate
imports of multiple TMX files if an error occurs, continue with the rest
of the files.

See #748.

src/prefsdlg.cpp

@@ -619,8 +619,12 @@ class TMPageWindow : public PrefsPanel
                             wxOK | wxICON_ERROR
                         ));
                     err->SetExtendedMessage(DescribeCurrentException());
-                    err->ShowWindowModalThenDo([err](int){});
-                    break;
+                    // FIXME: can't use ShowWindowModalThenDo, as would be better, because multiple
+                    //        errors may occur in this loop. See https://github.com/vslavik/poedit/issues/748
+                    if (paths.size() == 1)
+                        err->ShowWindowModalThenDo([err](int){});
+                    else
+                        err->ShowModal();
                 }
             }
             UpdateStats();