Add a single exit point

Keep track of whether the callback was called and throw an error if it
wasn't.
Employ 'it worked if it ends with ok'.
Handle errors better

Use `err` returned from `nscale-sdk`.
React to `error` and `end` events on the SDK object.
Don't try disconnecting from the server if we're not connected.

Fixes #16.
Use `error-handling` branch of `nscale-sdk`
Don't treat `stderr` output as error output

`stderr` is sometimes used for informative output.

Don't manipulate `stderr` line endings.
Check for response in `buildContainer`
Merge pull request #20 from nearform/nicer-logs

Nicer logs
Merge pull request #19 from nearform/error-handling

Error handling