Fix TypeError when exc_info=False

[stj]

Formatting of a message fails in the StdlibFormatter when the
exc_info parameter is False. The traceback shows that handling of
the boolean is not done correct in the formatting for error type and
message.

Traceback (most recent call last):
  File "lib/python3.8/logging/__init__.py", line 1081, in emit
    msg = self.format(record)
  File "lib/python3.8/logging/__init__.py", line 925, in format
    return fmt.format(record)
  File "ecs_logging/_stdlib.py", line 117, in format
    result = self.format_to_ecs(record)
  File "ecs_logging/_stdlib.py", line 163, in format_to_ecs
    value = extractors[field](record)
  File "ecs_logging/_stdlib.py", line 150, in <lambda>
    if (r.exc_info is not None and r.exc_info[0] is not None)
TypeError: 'bool' object is not subscriptable

This replaces the lambda with methods handling exc_info being an
boolean or iterable.

[apmmachine]

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• Start Time: 2021-07-06T16:36:02.320+0000

• Duration: 8 min 24 sec

• Commit: 19c9834

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[sethmlarson]

Thanks for this. Here's some comments for you, going to tag in other reviewers.

[sethmlarson]

jenkins test this please

[sethmlarson]

jenkins test this please

Changes incorporated.