Pull the filesystem error type definition outside the functor

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Add a pretty-printer for the filesystem_error type

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
fat: improve the error printing in the command-line tool

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Remove traces of the old (worse) error printer

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Fix the BLOCK error type as well as the FS error type

This makes the pretty-printer functions simpler, and we were already
depending on V1.BLOCK in the functor argument anyway.

Note we have duplicated the printer for the BLOCK error here. Perhaps
this should live in mirage-types?

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #26 from djs55/improve-error-type

Expose the error type(s), add pretty-printers and an fs.mli