Decide how to pass exceptions across the C API (tracking swss-common/#932)

[erer1243]

Main issue: sonic-net/sonic-swss-common#932

Once that issue is solved, we will need to implement that here.
Maybe it can look something like this?

// in swss-common
fn catch<T, Fn: FnOnce() -> T>(f: Fn) -> Result<T, String> {
  unsafe { SWSSTry_() };
  let ans = f();
  let err = unsafe { SWSSCatch() };
  if err.is_null() {
    Ok(ans)
  } else {
    Err(string_from_cstr(err))
  }
}

// in hamgrd/other library user code
let res = swss_common::catch(|| DBConnector::new(...));
match res {
  Ok(db_connector) => { ... }
  Err(err_msg) => { ... }
}