Fix escaped error message (#230)

- This specific message will [already be generated](https://codeberg.org/Codeberg/pages-server/src/commit/974229681f4cc7f1ed31df9b05eabef2df01380/html/error.go#L44) when `http.StatusMisdirectedRequest` is set as status with [an empty message](974229681f/html/error.go (L25-L28)).
- Resolves https://codeberg.org/Codeberg/pages-server/issues/228

Co-authored-by: Gusted <postmaster@gusted.xyz>
Reviewed-on: https://codeberg.org/Codeberg/pages-server/pulls/230
Reviewed-by: 6543 <6543@obermui.de>
Co-authored-by: Gusted <gusted@noreply.codeberg.org>
Co-committed-by: Gusted <gusted@noreply.codeberg.org>

server/handler/handler_custom_domain.go

@@ -49,7 +49,7 @@ func handleCustomDomain(log zerolog.Logger, ctx *context.Context, giteaClient *g
 	}, canonicalLink); works {
 		canonicalDomain, valid := targetOpt.CheckCanonicalDomain(giteaClient, trimmedHost, mainDomainSuffix, canonicalDomainCache)
 		if !valid {
-			html.ReturnErrorPage(ctx, "domain not specified in <code>.domains</code> file", http.StatusMisdirectedRequest)
+			html.ReturnErrorPage(ctx, "", http.StatusMisdirectedRequest)
 			return
 		} else if canonicalDomain != trimmedHost {
 			// only redirect if the target is also a codeberg page!