caddyhttp: Fix fallback for the error handler chain (#4131)

* caddyhttp: Fix fallback for the error handler chain

The fix I went with in the end (after realizing some mistaken assumptions in #4131) is to just make the routes fall back to errorEmptyHandler instead of the non-error empty handler, if Terminal is true, making the routes error-aware. Ultimately this was probably just an oversight when errors was implemented at some point in the early betas of v2.

See https://caddy.community/t/problem-with-basicauth-handle-errors/12243/9 for context.

* Revert "caddyhttp: Fix fallback for the error handler chain"

This reverts commit 95b6ac44a6.

* caddyhttp: Fix via `routes.go`

modules/caddyhttp/routes.go

@@ -220,7 +220,11 @@ func wrapRoute(route Route) Middleware {
 
 			// make terminal routes terminate
 			if route.Terminal {
-				nextCopy = emptyHandler
+				if _, ok := req.Context().Value(ErrorCtxKey).(error); ok {
+					nextCopy = errorEmptyHandler
+				} else {
+					nextCopy = emptyHandler
+				}
 			}
 
 			// compile this route's handler stack