use proper error variant on coin filtering

Signed-off-by: onur-ozkan <[email protected]>

mm2src/coins/rpc_command/tendermint/staking.rs

@@ -140,7 +140,7 @@ pub async fn validators_rpc(
                 .await?
         },
         Ok(_) => return MmError::err(ValidatorsRPCError::UnexpectedCoinType { ticker: req.coin }),
-        Err(_) => return MmError::err(ValidatorsRPCError::UnexpectedCoinType { ticker: req.coin }),
+        Err(_) => return MmError::err(ValidatorsRPCError::CoinNotFound { ticker: req.coin }),
     };
 
     Ok(ValidatorsRPCResponse {