Show errors from getSchedulerGroup().

If we failed to get the cgroup info, we were showing "unknown" in the
stack trace, and sometimes showing an error message in the log file.
Now we put a very brief error description into the cgroup text field,
so it shows up in the stack trace itself.

Also, replaced three strsep() calls with two strchr() calls.

Also, check return value from pthread_kill() calls.

Bug 3032532.

Change-Id: I3730cfb8a5c59048d35d746d8bffb4fa61bd5a6c