More descriptive error messages, for when Steam is not found

This should work on all platforms. 

The old error messages were vague and required digging into the logs and source code to find out what went wrong. The new messages say "Steam installation not found" on Windows, and "Steam userdata directory not found in the default location:" on Linux and OSX along with the path Ice looked in.

I think the user should be given the option to give Ice the correct path when one of these errors occurs, or quit. That should make it a little more user friendly while we don't have a GUI.