Explicitly check for no / in /proc/mounts
Explicitly check for no / in /proc/mounts

This doesn't solve any issues, but it does provide an explicit error
message when the /proc/mounts parser in euca-bundle-vol can't find a /
filesystem for whatever reason rather than simply terminating with a
"referenced before assignment" error.

See EUCA-636

gholms/euca2ools,gholms/euca2ools