SlowCallers: make fewer errors fatal
SlowCallers: make fewer errors fatal

Summary:
Callers() should be more tolerant of empty ticket
sets and missing data.

Reviewers: schroederc

Reviewed By: schroederc

Differential Revision: https://phabricator-dot-kythe-repo.appspot.com/D819

legrosbuffle/kythe,kythe/kythe,bzz/kythe,legrosbuffle/kythe,benjyw/kythe,bowlofstew/kythe,legrosbuffle/kythe,benjyw/kythe,bzz/kythe,kythe/kythe,benjyw/kythe,kythe/kythe,bowlofstew/kythe,bowlofstew/kythe,bowlofstew/kythe,bzz/kythe,kythe/kythe,legrosbuffle/kythe,bzz/kythe,bzz/kythe,legrosbuffle/kythe,benjyw/kythe,bzz/kythe,bowlofstew/kythe,bowlofstew/kythe,benjyw/kythe,bzz/kythe,kythe/kythe,bowlofstew/kythe,legrosbuffle/kythe,bowlofstew/kythe,benjyw/kythe,kythe/kythe,legrosbuffle/kythe,kythe/kythe,benjyw/kythe,legrosbuffle/kythe,kythe/kythe,legrosbuffle/kythe,bowlofstew/kythe,kythe/kythe,bowlofstew/kythe,bzz/kythe,bzz/kythe,benjyw/kythe,benjyw/kythe,benjyw/kythe,bowlofstew/kythe,benjyw/kythe,bzz/kythe,bzz/kythe,kythe/kythe,kythe/kythe,legrosbuffle/kythe,bzz/kythe,benjyw/kythe,legrosbuffle/kythe