aws ec2 preqreq check: match error to filter

filter to search for a subnet searches by both regionZone and VpcId,
but when no matches were found, the error message neglected to mention
that the VPC ID was part of the search criteria

Signed-off-by: F. Eugene Aumson <feuGeneA@github.com>

drivers/amazonec2/amazonec2.go

@@ -503,7 +503,7 @@ func (d *Driver) checkPrereqs() error {
 		}
 
 		if len(subnets.Subnets) == 0 {
-			return fmt.Errorf("unable to find a subnet in the zone: %s", regionZone)
+			return fmt.Errorf("unable to find a subnet that is both in the zone %s and belonging to VPC ID %s", regionZone, d.VpcId)
 		}
 
 		d.SubnetId = *subnets.Subnets[0].SubnetId