Use Manual.find_by_slug! in SectionReslugger
Use Manual.find_by_slug! in SectionReslugger

This removes some duplication regarding the look-up of the `Manual`
instance.

I've rescued the `Manual` exceptions and converted them into
`SectionReslugger::Error` exceptions in order to keep the interface the
same, although it's probably overkill.

alphagov/manuals-publisher,alphagov/manuals-publisher,alphagov/manuals-publisher