cli: gracefully handle errors during state data retrieval

In the event of a failure to fetch state data, merely print the error
received from holod instead of abruptly terminating the CLI.

Example:
```
holo# show state /invalid/path
% failed to fetch state data: status: InvalidArgument, message: "Failed to get operational data: invalid YANG data path" [snip]
holo#
```

Signed-off-by: Renato Westphal <[email protected]>

holo-cli/src/client/grpc.rs

@@ -178,7 +178,7 @@ impl Client for GrpcClient {
                 encoding: proto::Encoding::from(format) as i32,
                 path: xpath.unwrap_or_default(),
             })
-            .expect("Failed to parse gRPC Get() response")
+            .map_err(Error::Backend)?
             .into_inner()
             .data
             .unwrap();