Add more error information for invalid dictionaries

#33

Author: Krusen

BencodeNET/Parsing/BDictionaryParser.cs

@@ -80,10 +80,16 @@ public override BDictionary Parse(BencodeStream stream)
                 catch (BencodeException ex)
                 {
                     throw InvalidException(
-                        "Could not parse dictionary value. There needs to be a value for each key.",
+                        $"Could not parse dictionary value for the key '{key}'. There needs to be a value for each key.",
                         ex, startPosition);
                 }
 
+                if (dictionary.ContainsKey(key))
+                {
+                    throw InvalidException(
+                        $"The dictionary already contains the key '{key}'. Duplicate keys are not supported.", startPosition);
+                }
+
                 dictionary.Add(key, value);
             }
 
@@ -96,6 +102,12 @@ public override BDictionary Parse(BencodeStream stream)
             return dictionary;
         }
 
+        private static InvalidBencodeException<BDictionary> InvalidException(string message, long startPosition)
+        {
+            return new InvalidBencodeException<BDictionary>(
+                $"{message} The dictionary starts at position {startPosition}.", startPosition);
+        }
+
         private static InvalidBencodeException<BDictionary> InvalidException(string message, Exception inner, long startPosition)
         {
             return new InvalidBencodeException<BDictionary>(