slight rework of storage.mount logic; more info in messages

USB device mounting was not working on the test VM.  This change
was just a shuffling around of the code but now mounting works.
Speculation: The real reason may be a lengthening of the timeouts
in the mount/unmount commands in ddr (commit 16cf5bd740).

Author:    geoffrey jost <geoffrey.jost@densho.org>