Implement NoFail mod

osu.Game/Rulesets/Mods/Mod.cs

@@ -45,5 +45,10 @@ public abstract class Mod
         /// The mods this mod cannot be enabled with.
         /// </summary>
         public virtual Type[] IncompatibleMods => new Type[] { };
+
+        /// <summary>
+        /// Whether we should allow fails at the 
+        /// </summary>
+        public virtual bool AllowFail => true;
     }
 }

osu.Game/Rulesets/Mods/ModNoFail.cs

@@ -15,5 +15,10 @@ public abstract class ModNoFail : Mod
         public override double ScoreMultiplier => 0.5;
         public override bool Ranked => true;
         public override Type[] IncompatibleMods => new[] { typeof(ModRelax), typeof(ModSuddenDeath), typeof(ModAutoplay) };
+
+        /// <summary>
+        /// We never fail, 'yo.
+        /// </summary>
+        public override bool AllowFail => false;
     }
 }

osu.Game/Rulesets/Scoring/ScoreProcessor.cs

@@ -16,8 +16,9 @@ public abstract class ScoreProcessor
     {
         /// <summary>
         /// Invoked when the ScoreProcessor is in a failed state.
+        /// Return true if the fail was permitted.
         /// </summary>
-        public event Action Failed;
+        public event Func<bool> Failed;
 
         /// <summary>
         /// Invoked when a new judgement has occurred. This occurs after the judgement has been processed by the <see cref="ScoreProcessor"/>.
@@ -106,8 +107,8 @@ protected void UpdateFailed()
             if (alreadyFailed || !HasFailed)
                 return;
 
-            alreadyFailed = true;
-            Failed?.Invoke();
+            if (Failed?.Invoke() != false)
+                alreadyFailed = true;
         }
 
         /// <summary>

osu.Game/Screens/Play/Player.cs

@@ -238,13 +238,17 @@ private void onCompletion()
             }
         }
 
-        private void onFail()
+        private bool onFail()
         {
+            if (Beatmap.Value.Mods.Value.Any(m => !m.AllowFail))
+                return false;
+
             decoupledClock.Stop();
 
             HasFailed = true;
             failOverlay.Retries = RestartCount;
             failOverlay.Show();
+            return true;
         }
 
         protected override void OnEntering(Screen last)