Fix bug with too-long substrings (#350)

C#'s string.Substring method throws an exception if length is beyond
the end of the string, so we need to check the string length first.

src/LfMerge.Core/Reporting/ConversionError.cs

@@ -118,7 +118,8 @@ public string MongoId() {
 		}
 
 		public string Label() {
-			if (Comment == null || Comment.Content == null) return string.Empty;
+			if (Comment == null || String.IsNullOrEmpty(Comment.Content)) return string.Empty;
+			if (Comment.Content.Length < 100) return Comment.Content;
 			return Comment.Content.Substring(0, 100);
 		}
 	}