add go templating engine for error page and make errors more clear (#260)

ping #199
closes #213

Reviewed-on: https://codeberg.org/Codeberg/pages-server/pulls/260
Co-authored-by: crapStone <crapstone01@gmail.com>
Co-committed-by: crapStone <crapstone01@gmail.com>

server/handler/handler.go

@@ -65,7 +65,7 @@ func Handler(mainDomainSuffix, rawDomain string,
 		// Block blacklisted paths (like ACME challenges)
 		for _, blacklistedPath := range blacklistedPaths {
 			if strings.HasPrefix(ctx.Path(), blacklistedPath) {
-				html.ReturnErrorPage(ctx, "requested blacklisted path", http.StatusForbidden)
+				html.ReturnErrorPage(ctx, "requested path is blacklisted", http.StatusForbidden)
 				return
 			}
 		}