Previously, nvim sent a response with invalid request id (UINT64_MAX).
In functionaltests, catch unexpected error notifications in after_each().

Give embeders a chance to set up nvim, by processing a request before
startup. This allows an external UI to show messages and prompts from
--cmd and buffer loading (e.g. swap files)

close #8745

channel.c: WIP remove redundant method check and added FUNC_ATTR_NONNULL_ALL macro

channel.c channel_defs.h helpers.c: added Error field to RequestEvent, added no_op handler func

channel.c: use const char* instead of string and cleanup

channel.c; channel_defs.h; helpers.c: removed error from event again; send errors directly to the channel without using handlers and events

channel.c: fixed memory leak and lint errors

api/private/dispatch.c; api/vim.c; msgpack_rpc/channel.c msgpack_rpc/helpers.c added Error* field to msgpack_get_handler_for; further refactored channel.c

channel.c:323 changed order of evaluation in if statement

channel.c: removed superflous whitespace

dispatch.c: review comment

Fire autocmd when channel opens or its info changes.
Add a way for API clients can describe themselves.

Before this change, if $NVIM_LISTEN_ADDRESS was invalid, v:servername
was left empty.