🐛 Fixed "No default engine was specified and no extension was provided"
🐛 Fixed "No default engine was specified and no extension was provided"

refs #9681

- we already had a protection against these situations when serving the site (theme)
- it can happen that we have to initialise the express engine in the error handler in case the first request to /ghost produces an error (e.g. 503)
- otherwise the underlying error message is hidden and Ghost doesn't render the error html template correctly

dbalders/Ghost,bitjson/Ghost,Gargol/Ghost,janvt/Ghost,ErisDS/Ghost,Gargol/Ghost,Kikobeats/Ghost,TryGhost/Ghost,JohnONolan/Ghost,TryGhost/Ghost,e10/Ghost,JohnONolan/Ghost,JohnONolan/Ghost,e10/Ghost,Kikobeats/Ghost,kevinansfield/Ghost,kevinansfield/Ghost,TryGhost/Ghost,janvt/Ghost,ErisDS/Ghost,dbalders/Ghost,kevinansfield/Ghost,Kikobeats/Ghost,dbalders/Ghost,bitjson/Ghost,ErisDS/Ghost,chevex/undoctrinate