db.pm: removed debug line
ptrack: exit with non-zero exit code if command does not exist. Refs #2820
ptrack: changed progress and dump format. Refs #2820
event actions: Report error reason, when possible. Refs #2820

When a task has non-zero exit code the message can specify the error
reason.  If the message is empty, the reason should be provided by a
more specific sub-task.
ptrack: daemonize() and use syslog. Refs #2820

UI configuration is read from stdin in JSON format before fork()ing.
Merge branch 'b2820'