Fix resource import interrupted when item not valid.

Assets/Cryville/Crtr/Browsing/ResourceConverter.cs

@@ -12,6 +12,10 @@ namespace Cryville.Crtr.Browsing {
 			Name = StringUtils.EscapeFileName(name);
 		}
 		public string Name { get; private set; }
+		public abstract bool Valid { get; }
+		public override string ToString() {
+			return string.Format("{0} ({1})", Name, ReflectionHelper.GetSimpleName(GetType()));
+		}
 	}
 	public class ChartResource : Resource {
 		public ChartResource(string name, Chart main, ChartMeta meta) : base(name) {
@@ -19,17 +23,20 @@ namespace Cryville.Crtr.Browsing {
 		}
 		public Chart Main { get; private set; }
 		public ChartMeta Meta { get; private set; }
+		public override bool Valid { get { return true; } }
 	}
 	public class CoverResource : Resource {
 		public CoverResource(string name, FileInfo src) : base(name) {
 			Source = src;
 		}
 		public FileInfo Source { get; private set; }
+		public override bool Valid { get { return Source.Exists; } }
 	}
 	public class SongResource : Resource {
 		public SongResource(string name, FileInfo src) : base(name) {
 			Source = src;
 		}
 		public FileInfo Source { get; private set; }
+		public override bool Valid { get { return Source.Exists; } }
 	}
 }