The `headOption` operator returns the first element in `Source` wrapped
in `Some` or `None` in case when source is empty or failed e.g.:

  Source.empty[Int].headOption()       // None
  Source.fromValues(1, 2).headOption() // Some(1)

The `head` operator returns the first element in `Source` or throws
`NoSuchElementException` in case when it is either empty or `receive()`
operation fails without error. In case when `receive()` fails with
exception then this exception is re-thrown e.g.:

  Source.empty[Int].head()       // throws NoSuchElementException("cannot obtain head from an empty source")
  Source.fromValues(1, 2).head() // 1

Note that neither `head` nor `headOption` are idempotent operations.