Match all non-failed scores to D. This fixes low good attacks (eg. N goods,

two or three bads or misses, and no perfects or greats) which have a negative
grade percentage ending up at GRADE_FAILED (E).  It also fixes a potentially
confusing inconsistency with autoplay; if you miss the first note or two of a
song and autoplay the rest, your score is negativfe and you'd get an E; if
you autoplay immediately and have a 0 score, you'd get a D.

stepmania/src/PrefsManager.cpp

@@ -122,6 +122,7 @@ void PrefsManager::Init()
 	m_fGradePercent[GRADE_TIER_4] = 0.80f;	// A
 	m_fGradePercent[GRADE_TIER_5] = 0.65f;	// B
 	m_fGradePercent[GRADE_TIER_6] = 0.45f;	// C
+	m_fGradePercent[GRADE_TIER_7] = -99999;	// D
 	m_bGradeTier02IsAllPerfects = true;
 	
 	m_fSuperMeterPercentChangeMarvelous =	+0.05f;