don't fail build if no translation files exist

Makefile

@@ -22,9 +22,13 @@ messages.po: embroider*.py inkstitch.py
 locales:
 	# message files will look like this:
 	#   translations/messages-en_US.po
-	for po in translations/*.po; do \
-		lang=$${po%.po}; \
-		lang=$${lang#messages-}; \
-		mkdir -p locales/$$lang/LC_MESSAGES/; \
-		msgfmt $$po -o locales/$$lang/LC_MESSAGES/inkstitch.mo; \
-	done
+	if ls translations/*.po > /dev/null 2>&1; then \
+		for po in translations/*.po; do \
+			lang=$${po%.po}; \
+			lang=$${lang#messages-}; \
+			mkdir -p locales/$$lang/LC_MESSAGES/; \
+			msgfmt $$po -o locales/$$lang/LC_MESSAGES/inkstitch.mo; \
+		done; \
+	else \
+		mkdir locales; \
+	fi