Adds tests and makes sure errors are not swallowed

[achingbrain]

If a pull-stream source/through in the pipeline before a pull-sort step results in an error, the error is swallowed and an empty array is passed on to the next step.

This PR:

1. Ensures the error is passed on instead
2. Adds some tests for that & the happy path
3. Adds missing dependencies
4. Ensures this module won't pull in all of pull-stream as described in the pull-stream readme under minimal bundle.

[alanshaw]

🙏 please can we have merge & release?

[dominictarr]

thanks! great work. merged into 1.0.1

[dominictarr]

make that 1.0.2