dump: Drop pointless error_is_set(), DumpState member errp

dump: Drop pointless error_is_set(), DumpState member errp

In qmp_dump_guest_memory(), the error must be clear on entry, and we
always bail out after setting it, directly or via dump_init().
Therefore, both error_is_set() are always false.  Drop them.

DumpState member errp is now write-only.  Drop it, too.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>