#278 Setting install mode that doesn't exist should fail graceful

[jblake]

Setting an install mode which doesn't exist should either error gracefully or just use Standard no matter what they used.

For example, create a new installer, and add the /Y command-line argument yourself and point it to UninstallMode instead of InstallMode, set this <%UninstallMode%> to "Default".
Which will bomb in the uninstall since that mode means nothing.