RunnerOutput error type does not match langpack guide specification

[zeryx]

The RunnerOutput object is defined here:

langpacks/src/message.rs

Line 16 in b35efaa

Failure { error: ErrorMessage },

The Output section of bin/pipe makes no mention of an error key:
https://github.com/algorithmiaio/langpacks/blob/master/langpack_guide.md#binpipe

In the existing Python runtime, you can see a encapsulating error key that wraps the error object, here:
https://github.com/algorithmiaio/langpacks/blob/master/libraries/python3-runtime/context/pipe#L66-L77

[pmcq]

this is a bug in documentation - feel free to update the langpack_guide.md