Throw when anything is logged to stderr

[Yurickh]

The problem

Right now users are unable to identify when their cli is breaking other than passing debug: true.

Suggested solution

Either throw an error with the chunk that was fed into stderr, or stop pipe-ing stderr and pass it into process.stderr.
You might get into some kind of timing issues, since clifford doesn't run the command on itself.

Rationale

By throwing we notify the consumer that something is wrong, rather than silencing the error.

[Yurickh]

Alternatively we can provide a method to access their logs, akin to how we provide readLine and read.

[Yurickh]

Will be solved by unifying stderr and stdout under readLine, read and similar method in #16