fix _set_relation when no target eids, update datafeed source pull_data arguments to raise on error during tests

[session] enhance session's transaction storage handling to fix cases where commit/rollback is done while in the context of hooks_control/security_enabled managers. Closes #1412648: commit or rollback during postcreate reset hooks control state

[session] nicer local variable names

[web server] #1642893 issue a 403 instead of a 500 on Unauthorized exception To do so, we allow an arbitrary code to be passed to ``error_handler``. Default error code is still 500.