noux: don't detach regions if dataspace is invalid

The dataspace cap in rm_ression could be invalid - freed up in
noux/ram_session_component.h. Implicitly attached regions are also freed up
at core.

Don't try to detach the region twice, since it may already be reused by other
attachments.

Since there is no way to figure out whether a dataspace is valid beside trying
to do something with it, try to call Dataspace_client(ds) and catch the
ipc_error.

Fixes #485
noux: temporary buildbot fix

Issue #815