We should reuse the `channel.xhr_error_message` in `ui_report` to reduce
code duplication.
Also, I changed the type of `message` parameter to `string` instead of
`string | null` so that we do not need to alter the types of the functions
that depends on the return value of `xhr_error_message`.
This changes to not report any `call POST /json/users/me/presence`
spans, which we previously reported despite not including the inner
auto-instrumented HTTP spans.
This lets us simplify the long-ish ‘../../static/js’ paths, and will
remove the need for the ‘zrequire’ wrapper.
Signed-off-by: Anders Kaseorg <anders@zulip.com>