This makes stderr and exit callbacks work for rpc jobs

Closes #3364

Also adds one exception to linter rules:

    typedef struct {
      kvec_t(Object) stack;
    } EncodedData;

is completely valid (from the style guide point of view) code.

It appears that used msgpack library is not able to parse back message created 
by msgpack_rpc_from_object() if nesting level is too high, so log_server_msg now 
cares about msgpack_unpack_next() return value. Also error message from 
server_notifications_spec.lua is not readable if something is wrong (though at 
least now it does not crash when parsing deeply nested structures).

log_server_msg() in the test reports

    [msgpack-rpc] nvim -> client(1) [error]        "parse error"

This removes some stack overflows in new test regarding deeply nested variables.
Now in place of crashing vim_to_object/msgpack_rpc_from_object/etc it crashes
clear_tv with stack overflow.

Rename main loop variable from loop to main_loop