We again build documentation in case of proper source errors (#253)

and only refuse building it in case of "GHC server died (dummy error)".
We are in no position to decide if partial documentation should be built
or not. Let the user of our API decide --- he has access to all relevant
information, in particular, to errors.

IdeSession/Update/ExecuteSessionUpdate.hs

@@ -702,11 +702,14 @@ executeBuildDoc = do
           Nothing ->
             error "This session state does not admit artifact generation."
           Just Computed{computedErrors} -> toLazyList computedErrors
+        isDummyError err =
+          errorKind err == KindError
+          && errorMsg err == Text.pack "GHC server died (dummy error)"
     exitCode <-
-      if any (== KindError) $ map errorKind errors then do
+      if any isDummyError errors then do
         exceptionFree $ do
           writeFile beStderrLog
-            "Source or other errors encountered. Not attempting to build documentation."
+            "GHC server died. Not attempting to build documentation."
           return $ ExitFailure 1
       else exceptionFree $ do
                   (loadedMs, pkgs) <- buildDeps mcomputed