nbd/client: Use error_prepend() correctly

nbd/client: Use error_prepend() correctly

When using error prepend(), it is necessary to end with a space
in the format string; otherwise, messages come out incorrectly,
such as when connecting to a socket that hangs up immediately:

can't open device nbd://localhost:10809/: Failed to read dataUnexpected end-of-file before all bytes were read

Originally botched in commit e44ed99d, then several more instances
added in the meantime.

Pre-existing and not fixed here: we are inconsistent on capitalization;
some of our messages start with lower case, and others start with upper,
although the use of error_prepend() is much nicer to read when all
fragments consistently start with lower.

CC: [email protected]
Signed-off-by: Eric Blake <[email protected]>
Message-Id: <20171113152424[email protected]>
Reviewed-by: Vladimir Sementsov-Ogievskiy <[email protected]>
Reviewed-by: Markus Armbruster <[email protected]>