warn user if chimera cannot be found on any of the default paths

If chimera is selected as a structure viewer, Jalview should check to see if it can actually locate Chimera and if not, pop up a dialog box and/or help to provide instructions on downloading and installing the package.

Currently, it's possible to select 'Chimera' and open a view window, but Jalview silently fails - with the only indication that something went wrong being on the console output.