Fix #1421 Update SlackApiError exception handling for web client

[vinceta]

Summary

This pull request resolves #1421, also updated unit tests and README.

Category (place an x in each of the [ ])

• slack_sdk.web.WebClient (sync/async) (Web API client)
• slack_sdk.webhook.WebhookClient (sync/async) (Incoming Webhook, response_url sender)
• slack_sdk.socket_mode (Socket Mode client)
• slack_sdk.signature (Request Signature Verifier)
• slack_sdk.oauth (OAuth Flow Utilities)
• slack_sdk.models (UI component builders)
• slack_sdk.scim (SCIM API client)
• slack_sdk.audit_logs (Audit Logs API client)
• slack_sdk.rtm_v2 (RTM client)
• /docs-src (Documents, have you run ./scripts/docs.sh?)
• /docs-src-v2 (Documents, have you run ./scripts/docs-v2.sh?)
• /tutorial (PythOnBoardingBot tutorial)
• tests/integration_tests (Automated tests for this library)

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've run python3 -m venv .venv && source .venv/bin/activate && ./scripts/run_validation.sh after making the changes.

[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @vinceta to sign the Salesforce Inc. Contributor License Agreement.

[seratch]

Hi @vinceta, thanks a lot for sending this PR! Could you check the following points?

1. Signing the CLA

We are unable to merge your changes without signed CLA. Would you mind checking and signing it ?

2. Resolving the lint error

the err import is no longer used, so imported module name needs to be removed from the code you've touched.

/home/runner/work/python-slack-sdk/python-slack-sdk/slack_sdk/web/base_client.py:22:1: F401 'slack_sdk.errors as err' imported but unused
/home/runner/work/python-slack-sdk/python-slack-sdk/slack_sdk/web/base_client.py:469:13: F811 redefinition of unused 'err' from line 22
Installing test dependencies ...

https://github.com/slackapi/python-slack-sdk/actions/runs/6846854355/job/18614138792?pr=1423

[vinceta]

@seratch Thanks for your quick response! This's my first PR, apologies for any missing. Please see if any issues still exist, thanks.

[codecov]

Codecov Report

Merging #1423 (9001914) into main (2d592cd) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1423   +/-   ##
=======================================
  Coverage   85.43%   85.44%           
=======================================
  Files         111      111           
  Lines       11857    11857           
=======================================
+ Hits        10130    10131    +1     
+ Misses       1727     1726    -1     

Files	Coverage Δ
slack_sdk/web/base_client.py	89.62% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[seratch]

Looks great to me! Thanks for fixing this!

[seratch]

I will work on a patch release including this change tomorrow (or at latest as early as possible this week)