Print warnings when disabling write access

When the user has selected a verbose option, this seems like the kind of
thing they would be interested in.

Signed-off-by: Will Dyson <will.dyson@gmail.com>
Fail cleanly on volume open failure

When failing to open the volume (after read-only fallback), give an error
message and fail cleanly.

Signed-off-by: Will Dyson <will.dyson@gmail.com>
Merge pull request #5 from Thalience/err-messages

Improve error messages