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.***

I meant back before #81 was pulled.  Before that pull the error was always just returned with the stderror and the absorbed by the handler.  When we switched to using Spawn via BufferedProcess it started throwing this error because Spawn requires an executable of some sort while Exec can fail with just the stderror.   Anyway we should probably pull this soonish.