Comment 9 for bug 538292

plymouth -14 source has been accepted and will build shortly.

mountall 2.8 is published on i386, and is in the process of publishing on amd64. Installing this package is sufficient to fix the problem for any users who have run into this; the plymouth -14 just cleans up to make sure no one else has the problem.

https://bugs.launchpad.net/ubuntu/+source/plymouth/+bug/538292/comments/3 describes, in general terms, the process for recovering from this error. It is definitely not recommended to run pre-beta versions of Ubuntu without rescue media available to use for recovery!