+exception registration sugar

src/commonMain/kotlin/net.sergeych.parsec3/ExceptionsRegistry.kt

@@ -34,9 +34,10 @@ open class ExceptionsRegistry {
      * Register an exception with a code with a handler that creates its instance. Note that the
      * handler _should not throw anything_ but rather create an instance of the exception.
      */
-    inline fun <reified T : Throwable> register(code: String, noinline block: (message: String?) -> T) {
-        handlers[code] = block
-        classCodes[T::class] = code
+    inline fun <reified T : Throwable> register(code: String?=null, noinline block: (message: String?) -> T) {
+        val _code = code ?: T::class.simpleName!!
+        handlers[_code] = block
+        classCodes[T::class] = _code
     }
 
     /**