change signature of task function to return a Result

[jeremyandrews]

Currently Goose Task functions do not return anything, preventing us from using the ? shortcut to handle errors. This requires the use of more boilerplate code when writing load tests than is desirable. This issue is to implement the standard Rust pattern for returning a Result<T, E>.

Once implemented, from load test functions, on success we'll need to return Ok(), on failure we'll need to return Err().

[jeremyandrews]

fixed by #108