Arcyd: put log of system exceptions on dashboard

Make it easy to see where Arcyd has run into trouble by keeping a log of
system-level exceptions.

These are exceptions which aren't the result of user error, more the
result of problems in Arcyd's environment or implementation.

Test Plan:
$ ./precommit.sh
$ ./testbed/arcyd/status_html_testbed.sh

$ ./testbed/arcyd/loop_test.sh
- 'raise Exception("Whoops!")' where it will be noticed by the new
   calls to reporter.log_system_exception()