`bridge convert -o <dir>` unconditionally ran `os.RemoveAll` on the
output directory and discarded any error, so a typo (`-o .`, `-o
$PWD`, `-o ~`) silently destroyed unrelated user data.
An empty or missing output directory is still created silently, but a
non-empty one now requires explicit confirmation (prompted
interactively, or via the new `--yes` flag for scripts/CI) before
being wiped.
Signed-off-by: Guillaume Lours <glours@users.noreply.github.com>
