Fix #718 by handling unexpected response body format

[seratch]

Summary

This pull request fixes #718 by adding the logic to handle non-JSON response body data in WebClient (both sync and async). Although the situation almost never happens, raising only the errors defined in this SDK is more developer-friendly. Refer to the issue for details and use cases.

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.

[codecov]

Codecov Report

Merging #751 into main will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #751      +/-   ##
==========================================
+ Coverage   85.15%   85.19%   +0.03%     
==========================================
  Files          22       22              
  Lines        2803     2810       +7     
  Branches      281      283       +2     
==========================================
+ Hits         2387     2394       +7     
+ Misses        312      311       -1     
- Partials      104      105       +1     

Impacted Files	Coverage Δ
slack/web/base_client.py	72.27% <100.00%> (+0.61%)	⬆️

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