proto, server: Add internal server error message. (v2)

Many languages such as C++ or Rust have an unwinding error-reporting
mechanism. Code in these languages can (and must!) wrap request handling
callbacks in unwind guards to avoid undefined behaviour.

As a consequence such code will detect internal server errors, but have
no way to communicate such failures to the client.

This adds a WL_DISPLAY_ERROR_IMPLEMENTATION error to wl_display so that
such code can notify (and disconnect) clients which hit internal bugs.
While servers can currently abuse other wl_display errors for the same
effect, adding an explicit error code allows clients to tell the
difference between errors which are their fault and errors which are the
server's fault. This is particularly interesting for automated bug
reporting.

v2: Rename error from "internal" to "implementation", in sympathy with
    X11's BadImplementation error.
    Add more justification in the commit message.

Signed-off-by: Christopher James Halse Rogers <christopher.halse.rogers@canonical.com>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.com>