make can_transform code return bool

The current interface returns (bool, errorstr).
This is nice to have, but it's unintuitive and
provokes the mistake "not buffer.can_transform(..)"
which should really be "not buffer.can_transform(..)[0]".
Such a bug ( as currently in tf2_ros/buffer.py:72 )
is much worse than receiving an error string without
raising an exception.