Added stacktrace to error logs

Summary:
Error logs are not very useful without stacktrace information.

Test Plan:
Made a call to trigger_error and checked the error log file.

Reviewed By: jungejason
Reviewers: jungejason
Commenters: epriestley
CC: epriestley, aran, jungejason, tuomaspelkonen
Differential Revision: 234