Bail out early if no JS interpreter was found!

Danger! My system's got no JS!

At least some of the machines I'm working on don't have any JS interpreter installed. Sad but true. By shifting a few lines up to the beginning of the `jslint.vim` file and returning early if no interpreter was found, we get one concise error message ("No JS interpreter found. Checked for jsc, js (spidermonkey), and node") instead of this:

    Error detected while processing /home/jpommerening/.vim/plugin/jslint.vim:
    line   83:
    No JS interpreter found. Checked for jsc, js (spidermonkey), and node
    line   93:
    E121: Undefined variable: s:cmd
    E15: Invalid expression: 'cd "' . s:plugin_path . '" && ' . s:cmd . ' "' . s:plugin_path . 'runjslint.' . s:runjslint_ext . '"'
    Press ENTER or type command to continue