RFC - Add internal server error message

[raof at ubuntu.com]

Because C++ exceptions escaping into C FFI invokes undefined behaviour
our Wayland implementation wraps all the interface vtable callbacks
with try {} catch (...) { log_error }.

This has the nice property of not crashing the server, but does result
in the client and server having different ideas about the current state.

In the interests of debugability it would be nice to tell clients that
their request failed due to server error rather than silently continue
into unknown territory.

These patches introduce a WL_DISPLAY_ERROR_INTERNAL error the compositor
can send in such cases.