Setup error dialog and logging can be greatly improved

[caesay]

DisplayErrorMessage can be called with either no logFile, or with a path to a logFile that does not exist. In both of those cases, a modal is shown with the buttons Open Setup Log and Close - but clicking Open Setup Log does nothing.

Fix:

• If there is no log file provided, or the file does not exist, the Open Setup Log button should not appear.
• If Update.exe --install is the thing that failed, we should link up to that log file. (maybe, we specify a log file location via a command line argument when starting Update.exe?)
• Review logging in Setup.exe more generally - make sure it is working, being stored in a sensible location, has good coverage etc.

[caesay]

Setup.exe is now an extremely simple shim and does not require any logging. if an error occurs, it will alert the user with a message box containing the specific error that occurred. Setup.exe's only job is to extract Update.exe and run it - after which typical logging can take place.

Additionally, logging from Update.exe has been significantly improved. Rolling log files now ensure that log files never get too large, and get cleaned up after themselves (see 2299ecd)