GitLab

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.