decoder: Try to provide more explicit error context on invalid data (#29)

This partly reverts 572d0a6a and just changes %t -> %T so that printing
does not enter infinite recursion and just print what we need: type of
an object (%t is for true/false).

Along the way I've added "pickle:" prefix to errors.

In the future maybe it would make sense to create dedicated DecodeError
which would contain position, current opcode, and error message itself.
This way error reporting would be more structured and useful for users.

ogorek.go

@@ -87,7 +87,6 @@ var errNotImplemented = errors.New("unimplemented opcode")
 var ErrInvalidPickleVersion = errors.New("invalid pickle version")
 var errNoMarker = errors.New("no marker in stack")
 var errStackUnderflow = errors.New("pickle: stack underflow")
-var errMalformedData = errors.New("pickle: malformed data")
 
 type OpcodeError struct {
 	Key byte
@@ -618,7 +617,7 @@ func (d *Decoder) loadAppend() error {
 		l := l.([]interface{})
 		d.stack[len(d.stack)-1] = append(l, v)
 	default:
-		return errMalformedData
+		return fmt.Errorf("pickle: loadAppend: expected a list, got %T", l)
 	}
 	return nil
 }
@@ -683,7 +682,7 @@ func (d *Decoder) loadAppends() error {
 		}
 		d.stack = append(d.stack[:k-1], l)
 	default:
-		return errMalformedData
+		return fmt.Errorf("pickle: loadAppends: expected a list, got %T", l)
 	}
 	return nil
 }
@@ -834,7 +833,7 @@ func (d *Decoder) loadSetItem() error {
 		m := m.(map[interface{}]interface{})
 		m[k] = v
 	default:
-		return errMalformedData
+		return fmt.Errorf("pickle: loadSetItem: expected a map, got %T", m)
 	}
 	return nil
 }
@@ -856,7 +855,7 @@ func (d *Decoder) loadSetItems() error {
 		}
 		d.stack = append(d.stack[:k-1], m)
 	default:
-		return errMalformedData
+		return fmt.Errorf("pickle: loadSetItems: expected a map, got %T", m)
 	}
 	return nil
 }