stream output might break. in these cases we need to notify the
client. this is after a 200 response has been sent. We do this
by setting a special trailer (header after the body):

  X-Stream-Error: <error cause>

This is similar to what's done by systems like gRPC. This still
needs to be read + handled on the other side.

License: MIT
Signed-off-by: Juan Batiz-Benet <juan@benet.ai>