Instead of special-casing the callback, which is the path least-taken,
switch the internals to a Handler model, where the default behaviour is
an implementation of a Handler, and external callers can implement their
own Handlers and pass them in.
While doing so, change all unwraps in run::run to returning Errs, and
expand the watchexec Error enum to accommodate. That should make it
easier to use as a library.
Also, differentiate between "manual" and "on update" runs. For now the
only manual run is the initial run, but this paves the way for e.g.
keyboard- or signal- triggered runs.
