git-gui: display error launching blame as a message box.
git-gui: display error launching blame as a message box.

This does not appear to Windows users and can follow the form of the fatal
error messages near the top of the script file.

Signed-off-by: Pat Thoyts <fe43a83262eef1680511f524a1e74d43c633cffe@users.sourceforge.net>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git