refactor: avoid exception when lambda_handler envs not present in aws-lambda instrumentation

[Ronald-TR]

Description

This small refactor explains what happens when the aws-lambda instrumentation fails during _instrument() method, the exception occurs when the environment variables ORIG_HANDLER and _HANDLER doesn't exists, probably because the instrumentation is running outside an aws lambda.

Otherwise, this doesn't fix the original issue: opentelemetry-instrumentation-aws-lambda is installed by default during opentelemetry-bootstrap -a install, that is the reason why the original issue happens.

The PR just brings more clarity for this error.

Fixes #2744

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Test A

Does This PR Require a Core Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

See contributing.md for styleguide, changelog guidelines, and more.

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: Ronald-TR / name: Ronald (03e7c7d, 9371f41, b644506, 7a8b084, 630e0ab, 2add160, fcd10e7, 7dcb5ec, 0bff70e, 4be2056, 5b09a9f, 9819673, 9198b6e)
• ✅ login: lzchen / name: Leighton Chen (131c9ab)

[emdneto]

Quick example:

import requests

requests.get("https://httpbin.org/robots.txt")

Installs:

pip install --upgrade pip
pip install opentelemetry-distro
pip install requests
opentelemetry-bootstrap -a install

Run

opentelemetry-instrument --traces_exporter console --metrics_exporter console --service_name test python example.py

[xrmx]

LGTM with the text updated to give an hint on how to fix things