This is returned when the process you are looking for is not found. It's also used for other things that don't exist in the filesystem not being found too, for instance, if you try and delete a route from the routing table and that route doesn't exist, you'll get a confusing "Process not found" message, see route(8).
Look to recapture, but time marches away. That process is gone.
7 pages link to ESRCH:
lib/plugin/WlugLicense.php:99: Warning: Invalid argument supplied for foreach()
lib/plugin/WlugLicense.php:111: Warning: in_array() [<a href='function.in-array'>function.in-array</a>]: Wrong datatype for second argument