Catch invocation event tag extraction exceptions

[DylanLovesCoffee]

What does this PR do?

Moves self.response initialization and the logic to extract span tags from the event that invokes the Lambda function within a try...catch so it doesn't break the Lambda.

Motivation

In general, we shouldn't break the user's function from executing. In this case, if the invoking event is not what we expect, we can break the function from being executed. For example, an S3 event that doesn't have an arn field raised this error:

{
  "errorMessage": "'arn'",
  "errorType": "KeyError",
  "stackTrace": [
    "  File \"/opt/python/lib/python3.8/site-packages/datadog_lambda/wrapper.py\", line 55, in __call__\n    return self.func(*args, **kwargs)\n",
    ...
    "  File \"/opt/python/lib/python3.8/site-packages/datadog_lambda/trigger.py\", line 87, in parse_event_source_arn\n    return event_record.get(\"s3\")[\"bucket\"][\"arn\"]\n"
  ]
}

Catching the exception won't break the execution of the function and will still allow us to debug an issue.

Testing Guidelines

Passed unit tests and integration tests. Also tested my own build of the layer.

Additional Notes

Types of Changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
• This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• This PR impacts documentation, and it has been updated (or a ticket has been logged)
• This PR's changes are covered by the automated tests
• This PR collects user input/sensitive content into Datadog
• This PR passes the integration tests (ask a Datadog member to run the tests)