[FIXED JENKINS-23411] - Fix errors in ListView::doRemoveJobFromView() (CID 1222626)

* Throw an error if Jenkins cannot discover the item
* Synchronize delete&save operations from post requests

CID 1222626: https://scan.coverity.com/projects/94

Signed-off-by: Oleg Nenashev <o.v.nenashev@gmail.com>
Seems like the right thing to do, though not covered by test.