balrog agent fails to process subsequent changes if an exception is hit for an earlier one

[bhearsum]

We discovered this while testing https://bugzilla.mozilla.org/show_bug.cgi?id=1310226. Some of the scheduled changes used in testing would generate errors, because signoffs hadn't been given. When looking at sc_id 2, an error was generated (in that case, because signoffs weren't done), and that caused sc_id 3 to never be processed. We probably need to enhance the error handling in https://github.com/mozilla/balrog/blob/dc79a6b06ae1f38fd2d3fb20d8df20e5a7481d35/agent/balrogagent/cmd.py to continue in the two most inner loops if any exceptions are hit.

(Imported from https://bugzilla.mozilla.org/show_bug.cgi?id=1342191)