We should error check up front on the use of num_cache_nodes and
cluster_mode. This allows us to write a test to make sure all works as
expected
```
% make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSElasticacheReplicationGroup_clusteringAndCacheNodesCausesError'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/05/09 19:04:56 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSElasticacheReplicationGroup_clusteringAndCacheNodesCausesError -timeout 120m
=== RUN TestAccAWSElasticacheReplicationGroup_clusteringAndCacheNodesCausesError
--- PASS: TestAccAWSElasticacheReplicationGroup_clusteringAndCacheNodesCausesError (40.58s)
PASS
ok github.com/hashicorp/terraform/builtin/providers/aws 40.603s
```
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