feat: add actionable message on lint-locale failure (#12748)

Before this PR:
![image](/attachments/dd1a8f68-8b87-41e5-86e0-4ba59cedf4f4)

After this PR:
![image](/attachments/51007bbd-cf3c-4828-b5fc-9aecc9c958bd)

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/12748
Reviewed-by: 0ko <0ko@noreply.codeberg.org>
Reviewed-by: Michael Kriese <michael.kriese@gmx.de>
Reviewed-by: Ellen Εμίλια Άννα Zscheile <fogti@noreply.codeberg.org>

build/lint-locale/lint-locale.go

@@ -191,5 +191,24 @@ func main() {
 		}
 	}
 
+	if exitCode != 0 {
+		fmt.Println(dmp.DiffPrettyText([]diffmatchpatch.Diff{{
+			Type: diffmatchpatch.DiffEqual,
+			Text: "Please adjust the locale files as suggested above (",
+		}, {
+			Type: diffmatchpatch.DiffDelete,
+			Text: "red",
+		}, {
+			Type: diffmatchpatch.DiffEqual,
+			Text: ": removal, ",
+		}, {
+			Type: diffmatchpatch.DiffInsert,
+			Text: "green",
+		}, {
+			Type: diffmatchpatch.DiffEqual,
+			Text: ": insertion)",
+		}}))
+	}
+
 	os.Exit(exitCode)
 }