blockjob: fix user pause in block_job_error_action

Job (especially mirror) may call block_job_error_action several
times before actual pause if it has several in-flight requests.

block_job_error_action will call job_pause more than once in this case,
which lead to following block-job-resume qmp command can't actually
resume the job.

Fix it by do not increase pause level in block_job_error_action if
user_paused already set.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>