Better error message when the julia executable is not found

[waldyrious]

This is related to #18. I recently formatted my machine and amidst the various setup steps I forgot I hadn't yet reinstalled julia. As I went to install and configure Atom, the linter-julia plugin provided the "Failed to activate the linter-julia package" error, but nothing in the stack trace indicated what the underlying issue was. Only opening the linter-julia.coffee in the line indicated in the stack trace revealed that it failed in the jserver = spawn julia, ['-e', jcode] line.

It would be nice if the package told the user that the julia executable could not be found, rather than default to presenting the somewhat obscure stack trace.