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.***

It seems to me that there is no reason for the individual linter to be enabled if we silently swallow the failure to execute.   To be fair though, this is just restoring the original silent absorbing of this error.  So I'd say this is probably good to pull.