errors and async consequence

Hello,

the current API for errors is

```
errors(function(err, push) {})
```

so we can push one or more values at the current location of the stream if we already know those values.

When you need an async operation in order to know how to react to an error, how would you organize your streams ? a consume custom transform ? a sort of push(stream) + sequence

I wonder if there is room for an API of the style

```
errors(function(err, push, next) {})
```

The issue names the errors API and possible consume, custom-transform, push(stream), and sequence patterns, but no file or test. Start by tracing the errors API and comparing those async composition options; done requires an agreed callback contract and tests covering async error handling.

Written by the indexing model from the issue text.