#1357 Sync fails after changing sync folders

[eranl]

Commit 1562 seems to have introduced a bug, where sync fails after you change a sync folder, because it added a call to MessageBox.Show from the sync thread in that case.

Stack trace:
at System.Windows.Forms.Control.get_Handle()
at System.Windows.Forms.Control.GetSafeHandle(IWin32Window window)
at System.Windows.Forms.MessageBox.ShowCore(IWin32Window owner, String text, String caption, MessageBoxButtons buttons, MessageBoxIcon icon, MessageBoxDefaultButton defaultButton, MessageBoxOptions options, Boolean showHelp)
at System.Windows.Forms.MessageBox.Show(IWin32Window owner, String text, String caption, MessageBoxButtons buttons, MessageBoxIcon icon, MessageBoxDefaultButton defaultButton)
at GoContactSyncMod.SettingsForm.<sync_threadstarter>d__110.MoveNext()
in ...\GoogleContactsSync\SettingsForm.cs:line 911</sync_threadstarter>