fromSeconds: Update to user friendly error message

[razzpoker50]

Is your feature request related to a problem? Please describe.

When I was trying to get the timestamp in seconds DateTime.fromSeconds(modifiedDate) I am seeing the following error.

Uncaught Error: fromSeconds requires a numerical input

    at Function.fromSeconds (luxon.js:5344)

Describe the solution you'd like
It would be nice to have a better error message stating the type and the value that was passed in.
E.g.

throw new InvalidArgumentError(
        `fromSeconds requires a numerical input, but received a ${typeof seconds} with value ${seconds}`
      );

Describe alternatives you've considered
Additional guard checks in place.