DFG profiler should be helpful about gem installation
https://bugs.webkit.org/show_bug.cgi?id=104217

Reviewed by Oliver Hunt.
        
'json' and 'highline' are not installed by default on the ruby distributions with
which I am familiar, and the default error messages don't make me happy. This makes
display-profiler-output print a helpful message if those gems are not found.

* Scripts/display-profiler-output:



git-svn-id: svn://svn.chromium.org/blink/trunk@136881 bbb929c8-8fbe-4397-9dbb-9b2b20218538