Specify exposed capabilities for VCN gateway connection errors.

This CL updates VCN error callbacks to return the exposed capabilities
for a gateway connection for identification purposes. Previously, the
required underlying capabilities were returned (which are not meaningful
to the caller for identification purposes).

Bug: 182281888
Test: atest FrameworksVcnTests
Change-Id: Ic62bd0b524f24f2577d9abc7635112a6419d35ed