Improve handling of nagbar processes

Other changes in nagbar_exited:
- Remove ERROR from ELOG as it shows up as "ERROR: ERROR:"
- Make sure to reset the PID even when the process did not exit
normally.
- Use ELOG when exitcode != 0
- Remove the not found error: if nagbar is not in $PATH, exec_i3_utility
prints an error as well.

For the record, I also implemented a more complicated approach with a
new watcher data structure:
bd3aaf3a33
While writing the commit message, it occurred to me to compare
watcher->pid with *watcher->data, which fixes the problems mentioned in
that patch.

Fixes #4104

RELEASE-NOTES-next

@@ -42,3 +42,4 @@ working. Please reach out to us in that case!
   • build: correctly provide auxiliary functions when needed
   • build: fix issues with parallel build
   • set _NET_DESKTOP_VIEWPORT after randr changes
+  • fix a bug with i3-nagbar not starting after it has already started once