Make error on retrieval of nonexistent feature humanly readable

[mrzzy]

What this PR does / why we need it:
Notifies the user with a human readable error when the user attempts to retrieve
(both online/batch) feature data from feast serving with an feature reference does not match any feature stored in feast.

• currently the error that the user will see is:

InactiveRpcError: <_InactiveRpcError of RPC that terminated with:
	status = StatusCode.UNKNOWN
	details = ""
	debug_error_string = "{"created":"@1580607090.708289999","description":"Error received from peer ipv4:172.28.0.8:6566","file":"src/core/lib/surface/call.cc","file_line":1056,"grpc_message":"","grpc_status":2}"
>

• this PR replaces the the error with something more user friendly:

grpc.RpcError: Unable to find feature set for feature ref: (project: s, name: s, version: 1, max_age: )

Which issue(s) this PR fixes:

Fixes #454

Does this PR introduce a user-facing change?:

- Make error on retrieval of nonexistent feature humanly readable.

[woop]

project: s, name: s, version: 1, max_age:

Max age isn't a part of the feature reference, so I think that should be removed.

[woop]

/ok-to-test

[mrzzy]

/test test-serving

[mrzzy]

/test test-end-to-end

[mrzzy]

/test test-end-to-end-batch

[mrzzy]

@woop I have removed the max age from the error message.

[woop]

/lgtm
/approve

[feast-ci-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mrzzy, woop

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