mkinitfs: print error when failure to rm tmp dir, and make it non-fatal

This prints the error when the work dir can't be removed. This also changes mkinitfs so that it does't fail in this situation.

The reasoning for this change in behavior is that mkinitfs returning non-zero will signal to the caller that there's potentially a problem with configuring boot-related stuff on their system, and a failure to rm the work dir is just noise. If the cause of that failure is a deeper problem, the log message should help figure it out.

fixes #35 (closed)