Internal: streamline use of IndexClosedException introduced with #6475

Single index operations to use the newly added IndexClosedException introduced with #6475. This way we can also fail faster when we are trying to execute operations on closed indices and their use is not allowed (depending on indices options). Indices blocks are still checked but we can throw error earlier on while resolving indices (MetaData#concreteIndices).

Effectively this change also affects what we return when using one of the following apis: analyze, bulk, index, update, delete, explain, get, multi_get, mlt, term vector, multi_term vector. We now return: 
```
{"error":"IndexClosedException[[test] closed]","status":403}
``` 
instead of 
```
{"error":"ClusterBlockException[blocked by: [FORBIDDEN/4/index closed];]","status":403}
```

Closes #6988
cause this would be the only place where the constant would be used, while the other one was used in other places. Can add it though!