Add warning for invalid max-items value

[aemous]

Description of changes: Explicitly warn users that we do not support non-positive values for the max-items global pagination argument.

Example output:

aws s3api list-objects --bucket bucket-123 --max-items 0
warning: Non-positive values for --max-items may result in undefined behavior.

{
    "RequestCharged": null
} 

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.

Project coverage is 0.08%. Comparing base (f70e1e3) to head (8da209c). Report is 22 commits behind head on develop.

Files with missing lines	Patch %	Lines
awscli/customizations/paginate.py	0.00%	6 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           develop   #8931      +/-   ##
==========================================
- Coverage     0.08%   0.08%   -0.01%     
==========================================
  Files          210     210              
  Lines        16902   16911       +9     
==========================================
  Hits            14      14              
- Misses       16888   16897       +9     

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