Throw exception on TimeSpan parse failure

Forces command overloads which accept optional TimeSpan to be considered, instead of resolving to string with RemainingTextAttribute

BrackeysBot/ArgumentConverters/TimeSpanArgumentConverter.cs

@@ -63,6 +63,10 @@ public Task<Optional<TimeSpan>> ConvertAsync(string value, CommandContext ctx)
                     result += TimeSpan.FromSeconds(unitValue);
                     unitValue = 0;
                     break;
+
+                default:
+                    // force DisCatSharp to consider another overload
+                    throw new FormatException();
             }
         }