client: Ignore new requests if display has a fatal error

Once there has been a fatal display error, any new object requests
potentially rely on invalid state. (For example, a failure to read
from the compositor could hide a important event.) The safest way to
handle the new requests is not to make them.

Proxies produced by the request are still created, to ensure that
any code using the library does not crash from an unexpected NULL
pointer.

Signed-off-by: Manuel Stoeckl <code@mstoeckl.com>