Fix CantStartSingleInstanceException

It seems to be a .NET issue. I'd understand if it worked the other way round - so that the instance with admin privileges couldn't be given extra arguments by a normal instance. But as it is, it's probably a bug.
Probably the best we can do is just ignore the second instance and show a message.

See https://connect.microsoft.com/VisualStudio/feedback/details/782795/cannot-connect-to-previous-instance-when-running-as-administrator-using-windowsformsapplicationbase .

Fixes #274.