Transcoding Error Improvements

	raise ValueError(
	"Request {} does not match any URL path template in available HttpRule's {}".format(
	request_kwargs, [opt["uri"] for opt in http_options]
	)

The transcoding error above should be improved to provide more helpful information when there is a transcoding issue. For example, given the code below which is missing the instance_group parameter, the error message should say something along the lines of missing required parameter instance_group instead of the cryptic error Request {} does not match any URL path template in available HttpRule's ['/compute/v1/projects/{project}/zones/{zone}/instanceGroups/{instance_group}/listInstances']. In addition, it's not desirable to echo the parameters in the error message.

from google.cloud import compute_v1

project = "project-XXX"
zone = "us-central1-a"

instance_groups_list_instances_request_resource = compute_v1.InstanceGroupsListInstancesRequest()
instance_groups_list_instances_request_resource.instance_state = "RUNNING"
instance_groups_client = compute_v1.InstanceGroupsClient()

instance_groups_list = instance_groups_client.list_instances(project=project, zone=zone,  instance_groups_list_instances_request_resource=instance_groups_list_instances_request_resource)

for instance in instance_groups_list:
    print(instance.name, instance.labels)