Fixes functions outputJSON and pathExists execute callbacks even if there are errors/missing args

[emarteca]

Fixes #865

And, adds tests for this new error checking.

[RyanZim]

I'm not keen to duplicate business logic from universalify into every function body; the fact that fs does type checking of arguments is unusual for Node.js, but is necessary since Node core is backed by native code. I don't see the behavior of outputJSON as problematic personally; async functions generally transmit their errors by calling callbacks anyhow.

The pathExists behavior does seem to be a bug, since no error is thrown if only a callback is passed. pathExists should error out if no path is passed to it. I don't particularly care whether the error is thrown or given to the callback, but there should be an error. This also affects using pathExists as a promise; pathExists() should not resolve to false, as it does currently.