Deal with exc_info not being a tuple while not being None either in JsonLogFormatter

[diox]

JsonLogFormatter does:

python-dockerflow/src/dockerflow/logging.py

Lines 125 to 128 in d7fadb7

	# If there is an error, format it for nice output.
	if record.exc_info is not None:
	fields["error"] = repr(record.exc_info[1])
	fields["traceback"] = safer_format_traceback(*record.exc_info)

However, python logging docs say:

If exc_info does not evaluate as false, it causes exception information to be added to the logging message. If an exception tuple (in the format returned by sys.exc_info()) or an exception instance is provided, it is used; otherwise, sys.exc_info() is called to get the exception information.

The difference is notable, because there is code out there that calls log.something(..., exc_info=False). In AMO, which has a logger inheriting from JsonLogFormatter, that causes the following traceback:

  File "/usr/local/lib/python3.6/logging/__init__.py", line 994, in emit
    msg = self.format(record)
  File "/usr/local/lib/python3.6/logging/__init__.py", line 840, in format
    return fmt.format(record)
  File "/deps/lib/python3.6/site-packages/dockerflow/logging.py", line 138, in format
    out = self.convert_record(record)
  File "/code/src/olympia/core/logger.py", line 58, in convert_record
    out = super().convert_record(record)
  File "/deps/lib/python3.6/site-packages/dockerflow/logging.py", line 127, in convert_record
    fields["error"] = repr(record.exc_info[1])
TypeError: 'bool' object is not subscriptable