[IM] Encode error status code during command dispatch.

[erjiaqing]

Problem

We does not encode error code during dispatching commands.

Summary of Changes

Add code for encoding command responses.

The second commit is generated by

./scripts/tools/zap_regen_all.py && for x in $(gd --name-only | grep -ve '(.zap$)|(.py$)|(.gn$)'); do clang-format -i $x; done && git add examples/ src/ && gc -m "Run codegen"

[erjiaqing]

@bzbarsky-apple @mspang PTAL

[LuDuda]

LGTM 👍

[yunhanw-google]

@mspang @bzbarsky-apple ?

[erjiaqing]

@mspang @bzbarsky-apple conflict resolved, PTAL.