[core] Refactor how PythonGcsClient treats errors

[rynewang]

C++ GcsClient and PythonGcsClient (and python bindings in raylet_.pyx) are not very consistent. This PR aims to change the behavior of the latter to close to the former.

Behavior Changes

Situation	C++ Behavior	Python GcsClient Behavior	New Python GcsClient Behavior (this PR)
Timeout	ray::Status::TimedOut()	raise RpcError(rpc_code=DEADLINE_EXCEEDED)	raise RpcError(rpc_code=DEADLINE_EXCEEDED)
gRPC Issue (Unknown or Unavailable)	Retry the call or invoke Disconnected()	raise RpcError(rpc_code=the code)	raise RpcError(rpc_code=the code)
Other Status	Return status	raise RpcError(rpc_code=the code)	raise the error mapped from status
Payload Error	Return reply.status	raise InvalidError, or special treatment (see below)	raise the error mapped from reply.status, mostly RaySystemError
OK	Return OK	OK	OK

Payload Error Special Treatments

Method	reply.status	Python GcsClient Behavior	New Python GcsClient Behavior (this PR)
InternalKV(Multi)Get	KeyError (key invalid)	return None	return None
InternalKV(Multi)Get	NotFound (key valid but not found)	return None	raise RaySystemError

PythonGcsClient mostly just return an RpcError for everything without raising a more specific error type. With this PR we will do some error type mapping and return any non OK status as returned from the server side.

Implementation changes:

• Removed many manual error treating code and their quirks in PythonGcsClient.
• New error treatment function HandleGcsStatuses, which behaves similar to that in the C++ GcsRpcClient. The only difference is that we don't do any retry or return Disconnected.
• ray::Status::GrpcUnknown can carry a grpc code. To expose that rpc_code to Python, allow its ctor to accept a code.
• Fixed all tests.

All these should be OK, because GcsClient is not a public API and we can change its internal APIs arbitrarily, as long as all Ray call sites are handled.

[jjyao]

GetTimeoutError is for ray.get()?

I think RpcError(rpc_code=DEADLINE_EXCEEDED) is the one we want to raise