Set network's subnetwork.

When we're not using a default network, let's make sure we pick the
right subnetwork based on the region or raise an exception if one does
not exists..

Bug: 113535553
Test: acloud create with default and non-default network.
Change-Id: I8fba1ff1b0e2e8b8515d0b96aaf0e0e41b25bf20
diff --git a/public/errors.py b/public/errors.py
index 77ad0db..eb003f1 100755
--- a/public/errors.py
+++ b/public/errors.py
@@ -81,6 +81,10 @@
     """To catch device boot errors."""
 
 
+class NoSubnetwork(DriverError):
+    """When there is no subnetwork for the GCE."""
+
+
 class DeviceBootTimeoutError(DeviceBootError):
     """Raised when an AVD defice failed to boot within timeout."""