Kernel/Net: Propagate proper errno codes from determine_network_device

Returning literal strings is not the proper action here, because we
should always assume that error could be propagated back to userland, so
we need to keep a valid errno when returning an Error.

Kernel/Net/NetworkingManagement.cpp

@@ -117,7 +117,8 @@ UNMAP_AFTER_INIT ErrorOr<NonnullLockRefPtr<NetworkAdapter>> NetworkingManagement
  return adapter;
  }
  }
- return Error::from_string_literal("Unsupported network adapter");
+ dmesgln("Networking: Failed to initialize device {}, unsupported network adapter", device_identifier.address());
+ return Error::from_errno(ENODEV);
 }
 
 bool NetworkingManagement::initialize()