core: Don't return error on RegisterModule() and RegisterAdapter()

These functions are called at init-time, and their inputs are hard-coded
so there are no environmental or user factors that could make it fail
or succeed; the error return values are often ignored, and when they're
not, they are usually a fatal error anyway. To ensure that a programmer
mistake is not missed, we now panic instead.

Last breaking change 🤞

modules/caddyhttp/app.go

@@ -30,10 +30,7 @@ import (
 )
 
 func init() {
-	err := caddy.RegisterModule(App{})
-	if err != nil {
-		caddy.Log().Fatal(err.Error())
-	}
+	caddy.RegisterModule(App{})
 }
 
 // App is a robust, production-ready HTTP server.

modules/caddyhttp/caddyhttp.go

@@ -30,10 +30,7 @@ import (
 func init() {
 	weakrand.Seed(time.Now().UnixNano())
 
-	err := caddy.RegisterModule(tlsPlaceholderWrapper{})
-	if err != nil {
-		caddy.Log().Fatal(err.Error())
-	}
+	caddy.RegisterModule(tlsPlaceholderWrapper{})
 }
 
 // RequestMatcher is a type that can match to a request.