exceptions.py: Let Server504Error and Server414Error inherit from ServerError
exceptions.py: Let Server504Error and Server414Error inherit from ServerError

This is potentially a breaking change, but it makes sense and hopefully nobody
relies on catching a `ServerError` without catching `Server414Error` or
`Server504Error`.

Bug: T173289
Change-Id: I5f068886442d55aa0072815f4edbffbefa3577ae

magul/pywikibot-core,wikimedia/pywikibot-core,PersianWikipedia/pywikibot-core,happy5214/pywikibot-core,magul/pywikibot-core,happy5214/pywikibot-core,wikimedia/pywikibot-core