[chef_fs/file_system] Ignore missing entry at destination when purging

If a user attempts to call #copy_to with a path that does not exist at
either the source or destination and :purge set to true, we currently
fail with an error trying to delete the non-existent entry at the
destination.

This patch ignores that failure.
Merge pull request #1519 from opscode/ssd/purge-ignore-missing-on-purge

[chef_fs/file_system] Ignore missing entry at destination when purging