object-storage: Check for cancellation before resulting in assert failure

As per doumentation: "It is a programming error to create an identical proxy
while asynchronously creating one. Not cancelling this operation will result in
an assertion failure when calling cc_object_storage_create_dbus_proxy_finish()."
In order to fullfill the second part we need to check for errors (including
cancellation ones) before we generate an assertion failure.

Fixes: GNOME/gnome-control-center#158
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