Marshal Error.t as Backend_error(SCRIPT_FAILURE, [ Error.to_string_hum ])

If the script doesn't exist you get:

$ sm-cli query --queue org.xen.xcp.storage.test
Error from storage backend:
SCRIPT_FAILURE: [ (Unix.Exit_or_signal (Exit_non_zero 254)) ]

Signed-off-by: David Scott <dave.scott@citrix.com>