bencode: Improve UnmarshalTypeError string and list parsing error context

Helps with #297.

bencode/api.go

@@ -46,8 +46,7 @@ type UnmarshalTypeError struct {
 }
 
 func (e *UnmarshalTypeError) Error() string {
-	return "bencode: value (" + e.Value + ") is not appropriate for type: " +
+	return fmt.Sprintf("cannot unmarshal a bencode %s into a %s", e.Value, e.Type)
-		e.Type.String()
 }
 
 // Unmarshaler tried to write to an unexported (therefore unwritable) field.

bencode/decode.go

@@ -346,10 +346,10 @@ func (d *Decoder) parseList(v reflect.Value) error {
 	switch v.Kind() {
 	case reflect.Array, reflect.Slice:
 	default:
-		panic(&UnmarshalTypeError{
+		return &UnmarshalTypeError{
-			Value: "array",
+			Value: "list",
 			Type:  v.Type(),
-		})
+		}
 	}
 
 	i := 0