Merge remote branch 'mtodd/create_volume-raise-on-non-existent-snapshot'

lib/fog/compute/requests/aws/create_volume.rb

@@ -38,6 +38,8 @@ module Fog
         def create_volume(availability_zone, size, snapshot_id = nil)
           response = Excon::Response.new
           if availability_zone && size
+            raise Fog::AWS::Compute::NotFound.new("The snapshot '#{snapshot_id}' does not exist.") if snapshot_id && !@data[:snapshots][snapshot_id]
+
             response.status = 200
             volume_id = Fog::AWS::Mock.volume_id
             data = {