git-gui: Provide fatal error if library is unavailable

git-gui: Provide fatal error if library is unavailable

If we cannot locate our git-gui library directory, or we find it
but the tclIndex file is not present there (or it is present but
is not something we are allowed to read) the user cannot use the
application.  Rather than silently ignoring the errors related to
the tclIndex file being unavailable we report them up front and
display to the user why we cannot start.

Signed-off-by: Shawn O. Pearce <redacted>