Rewrite sequence linting / missing file handling.

- Distinguish between missing sequences and missing sprites
- Lint default sequences as well as maps
- Improved performance
- Correctly handle null images

OpenRA.Mods.Common/Projectiles/GravityBomb.cs

@@ -20,11 +20,11 @@ namespace OpenRA.Mods.Common.Projectiles
 	{
 		public readonly string Image = null;
 
-		[SequenceReference("Image")]
+		[SequenceReference("Image", allowNullImage: true)]
 		[Desc("Loop a randomly chosen sequence of Image from this list while falling.")]
 		public readonly string[] Sequences = { "idle" };
 
-		[SequenceReference("Image")]
+		[SequenceReference("Image", allowNullImage: true)]
 		[Desc("Sequence to play when launched. Skipped if null or empty.")]
 		public readonly string OpenSequence = null;