The onError callback can use the same future as the onResult callback by
calling completeExceptionally.
This change is also removing the test for a specific exception message.
Testing for a specific exception message would mean that tests break if we
ever were to update them. If we decide that there is value to
distinguish between different types of
EthernetNetworkManagementException we should probably subclass it
internally.
Test: atest EthernetNetworkFactoryTest
Change-Id: Ib6f8cf79fc26e0f856c880cbe505b101f12aa187
fd87fb65fb