docs: add call response error handling section (#824)

add error handling section for the Call response page in the docs

docs/src/calling-contracts/call-response.md

@@ -16,3 +16,21 @@ Where `value` will hold the value returned by its respective contract method, re
 
 - `receipts` will hold all [receipts](https://github.com/FuelLabs/fuel-specs/blob/master/specs/protocol/abi.md#receipt) generated by that specific contract call.
 - `gas_used` is the amount of gas it consumed by the contract call.
+
+## Error handling
+
+You can use the `is_ok` and `is_err` methods to check if a contract call `Result` is ok or contains an error. These methods will return either `true` or `false`.
+
+```rust, ignore
+let is_ok = response.is_ok();
+let is_error = response.is_err();
+```
+
+If `is_err` returns `true`, you can use the `unwrap_err` method to unwrap the error message.
+
+```rust, ignore
+if response.is_err() {
+    let err = response.unwrap_err();
+    println!("ERROR: {:?}", err);
+};
+```