Report failures to mount in localfs correctly.

The wrong exception type was being thrown, which meant that the virt
disk api though the disk could be resized when it couldn't. I've
added two unit tests to cover regressions as well, but this code needs
more unit testing in general. Resolves bug 1094373.

Change-Id: I9c974e138ff90e8b7a5a40f5b31dcdb25a59622d
Merge "Report failures to mount in localfs correctly."