Add support for different Maven POM encoding (#25873) (#25890)

Backport #25873 by @KN4CK3R

Fixes #25853

- Maven POM files aren't always UTF-8 encoded.
- Reject the upload of unparsable POM files

Co-authored-by: KN4CK3R <admin@oldschoolhack.me>

routers/api/packages/maven/maven.go

@@ -49,6 +49,11 @@ var (
 
 func apiError(ctx *context.Context, status int, obj any) {
 	helper.LogAndProcessError(ctx, status, obj, func(message string) {
+		// The maven client does not present the error message to the user. Log it for users with access to server logs.
+		if status == http.StatusBadRequest || status == http.StatusInternalServerError {
+			log.Error(message)
+		}
+
 		ctx.PlainText(status, message)
 	})
 }
@@ -326,7 +331,8 @@ func UploadPackageFile(ctx *context.Context) {
 		var err error
 		pvci.Metadata, err = maven_module.ParsePackageMetaData(buf)
 		if err != nil {
-			log.Error("Error parsing package metadata: %v", err)
+			apiError(ctx, http.StatusBadRequest, err)
+			return
 		}
 
 		if pvci.Metadata != nil {