combine: add _pipeline so errors will be handled

Previously, anytime streams that are passed to `combine` throw or emit an error, the error would not be caught. This change ensures that those errors are caught and bubbled up to the stream returned by `combine` (and further to any streams that stream is piped to), provided that the streams are run via Understream's `run` function.
edbc1c49f748bf514cf6143c9dc8f1346c922f6d