Better handling of invalid .ficfg files

Today I upgraded our test system to FI 2016.9. When I copied settings from DefaultSettings.ficfg into our config, I accidentally added two closing xml tags.

This invalid ficfg file caused a crash of FixInsight with an access violation in FixInsight.Services.dll. It was easy enough to solve, but a proper error message like "the configuration file '%s' is not a valid xml file" would be helpful.