new console command 'error' (stderr equivalent of console window)

Former-commit-id: ab54663fd64f9ae9c758f53b3f5800a5894f0db3
Former-commit-id: 7aba1585ffa45195622bb25e1c62cace474420c9

src/net/torvald/terrarum/console/Batch.kt

@@ -19,6 +19,6 @@ class Batch : ConsoleCommand {
     }
 
     override fun printUsage() {
-        Echo().execute("batch path/to/batch.txt")
+        Echo().execute("Usage: batch path/to/batch.txt")
     }
 }