logging errors on balrog agent

[bhearsum]

While investigating something else, we noticed this in the balrog agent logs (the "'bool' object is not subscriptable part is highly suspicious):

--- Logging error ---
Traceback (most recent call last):
File "/usr/local/lib/python3.5/logging/init.py", line 986, in emit
msg = self.format(record)
File "/usr/local/lib/python3.5/logging/init.py", line 836, in format
return fmt.format(record)
File "/app/balrogagent/log.py", line 83, in format
fields["error"] = repr(record.exc_info[1])
TypeError: 'bool' object is not subscriptable
Call stack:
File "/usr/local/lib/python3.5/runpy.py", line 193, in _run_module_as_main
"main", mod_spec)
File "/usr/local/lib/python3.5/runpy.py", line 85, in _run_code
exec(code, run_globals)
File "/app/balrogagent/cmd.py", line 113, in
main()
File "/app/balrogagent/cmd.py", line 107, in main
os.environ["TELEMETRY_API_ROOT"]
File "/usr/local/lib/python3.5/asyncio/base_events.py", line 454, in run_until_complete
self.run_forever()
File "/usr/local/lib/python3.5/asyncio/base_events.py", line 421, in run_forever
self._run_once()
File "/usr/local/lib/python3.5/asyncio/base_events.py", line 1425, in _run_once
handle._run()
File "/usr/local/lib/python3.5/asyncio/events.py", line 127, in _run
self._callback(*self._args)
File "/usr/local/lib/python3.5/asyncio/tasks.py", line 321, in _wakeup
self._step()
File "/usr/local/lib/python3.5/asyncio/tasks.py", line 240, in _step
result = coro.send(None)
File "/app/balrogagent/cmd.py", line 29, in run_agent
auth=auth, loop=loop)
File "/app/balrogagent/client.py", line 32, in request
resp = await aiohttp.request(method, url, data=json.dumps(data), headers=headers, auth=auth, loop=loop)

(Imported from https://bugzilla.mozilla.org/show_bug.cgi?id=1448392)