[splittiming] Broadcast message on failure.

Themes/_fallback/Languages/en.ini

@@ -1257,6 +1257,7 @@ This change creates more than %d notes in a measure.=This change creates more th
 This change creates notes past the end of the music and is not allowed.=This change creates notes past the end of the music and is not allowed.
 This will destroy all unsaved changes.=This will destroy all unsaved changes.
 Undo=Undo
+You must be in Step Timing Mode to edit the notes.=You must be in Step Timing Mode to edit the notes.
 
 Current beat=Current beat
 Current second=Current second

src/ScreenEdit.cpp

@@ -1276,6 +1276,7 @@ static int FindAttackAtTime( const AttackArray& attacks, float fStartTime )
 	return -1;
 }
 
+static LocalizedString TAP_NOTE_SONG_TIMING	( "ScreenEdit", "You must be in Step Timing Mode to edit the notes." );
 static LocalizedString SWITCHED_TO		( "ScreenEdit", "Switched to" );
 static LocalizedString NO_BACKGROUNDS_AVAILABLE	( "ScreenEdit", "No backgrounds available" );
 static ThemeMetric<bool> INVERT_SCROLL_BUTTONS	( "ScreenEdit", "InvertScrollSpeedButtons" );
@@ -1309,8 +1310,9 @@ void ScreenEdit::InputEdit( const InputEventPlus &input, EditButton EditB )
 			
 			if( !GAMESTATE->m_bIsEditorStepTiming )
 			{
-				// TODO: broadcast a message...?
+				// Only allow steps to be hit in Step Timing.
-				break; // Only allow steps to be hit in Step Timing.
+				SCREENMAN->SystemMessage( TAP_NOTE_SONG_TIMING );
+				break;
 			}
 			
 			int iCol = EditB - EDIT_BUTTON_COLUMN_0;