poudriere blocked after pkg build failed

[Tatsuki Makino]

For example, re-extracting ports tree.

> portsnap -d "`cat /usr/local/etc/poudriere.d/ports/default/mnt`/.snap"
-p "`cat /usr/local/etc/poudriere.d/ports/default/mnt`" extract

For example, to revert ports tree.

svnlite revert -R "`cat /usr/local/etc/poudriere.d/ports/default/mnt`"