Silently disable the linter if its command does not exist

More details about this bug: AtomLinter/linter-shellcheck#3 

I resolve this bug by silently disable the linter if its command does not exist. I think there should be a better way, for example, pop up a dialog to let the user choose disable the linter, or specify the linter command path manually.

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***The current commit (5c26e8e) resolve the performance issue. Please review.***

When this fails can we call to disable the specific linter package using the package manager.  It might be tricky.  However, if the user checks his packages, he will see that the linter is disabled and maybe check the console to see why.

https://atom.io/docs/api/v0.103.0/api/classes/PackageManager.html#disablePackage-instance