commandExists should return boolean instead of rejecting

[vihanb]

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Unless there was an error with checking if the command exists. It would make more sense and be more idiomatic to have a resolve(true/false) rather than rejecting in the case of the command not existing. This also means you can use async/await syntax: const exists = await commandExists('the-command')

[Huulivoide]

In addition this currently throw null as an error. Which if not handled locally, will tell absolutely nothing when caught in general error handler.

[mathisonian]

Thanks @vihanb @Huulivoide. I don't have time to implement this change at the moment, but would accept a PR for it.

[raftario]

You can check my fork that does what you want : https://github.com/raftario/command-exists The changes were too significant for me to open a PR, and it's not compatible with Node <6, so I decided to publish it as a separate module.