Raise build-time error on incomplete patterns

I realized I never intend to leave patterns (e.g. pattern matching in a 'case'
clause) incomplete, i.e. some cases left missing. When I do that it means I
forgot, and I'd like GHC to highlight it by raising an error instead of just
warning. Vervis has lots of warnings so it's hard to detect among them.

I suppose in other kinds of software people sometimes leave incomplete patterns
intentionally / are okay with a runtime exception being thrown? In a web
application, I definitely want to handle all cases, and be in control of how
errors are handled and displayed in UI.

src/Vervis/API.hs

@@ -922,6 +922,8 @@ createNoteC (Entity pidUser personUser) sharerUser summary audience note muTarge
                 if u == u'
                     then Nothing
                     else Just $ Right u'
+        checkParent _            _                           =
+            error "A situation I missed in pattern matching, fix it?"
     checkFederation remoteRecips = do
         federation <- asksSite $ appFederation . appSettings
         unless (federation || null remoteRecips) $

src/Vervis/ActivityPub.hs

@@ -1302,6 +1302,7 @@ getOutboxActorEntity obid = do
         (Just p, Nothing, Nothing) -> return $ ActorPerson p
         (Nothing, Just j, Nothing) -> return $ ActorProject j
         (Nothing, Nothing, Just r) -> return $ ActorRepo r
+        _ -> error "obid used by multiple actors"
 
 actorEntityPath (ActorPerson (Entity _ p)) =
     LocalActorSharer . sharerIdent <$> getJust (personIdent p)

vervis.cabal

@@ -399,7 +399,7 @@ library
 
   if flag(dev) || flag(library-only)
     cpp-options:       -DDEVELOPMENT
-    ghc-options:       -Wall -fwarn-tabs -O0
+    ghc-options:       -Wall -fwarn-tabs -O0 -Werror=incomplete-patterns
   else
     ghc-options:       -Wall -fwarn-tabs -O2