An example value of sys.retrieve.error is as follows:

[itctabuild02] ~ > sudo eos attr ls /eos/dev/userfiles/testdir_1/test_1 | grep sys.retrieve.error
sys.retrieve.error="Garbage collected at 2019/09/09 16:27:26.609570"
[itctabuild02] ~ >

cta-fst-gc now logs an error if it cannot list the contents of an FST filesystem or if determine the filesystem's sub directories

ctafstgcd.py no longer logs an ERROR when it fails to stat() a file that has been deleted by the FST

cta-fst-gcd now reports every 10,000 files that it has considered them.  This should help us see how fast the garbage collector is