fix: handle more errors with mailbox listing

[firstof9]

Proposed change

Fix bool error message in config flow.
Handle more errors with mailbox listing.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New feature (which adds functionality)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests
• Documentation update
• Adds a new shipper
• Update existing shipper

Additional information

• This PR fixes or closes issue: fixes ISSUE: Configuring multiple carriers causes integration to not initialize  #934

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 25.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 94.19%. Comparing base (8608d22) to head (4b227f2).
Report is 13 commits behind head on dev.

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Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #935      +/-   ##
==========================================
- Coverage   95.07%   94.19%   -0.88%     
==========================================
  Files           8        8              
  Lines        1482     1534      +52     
==========================================
+ Hits         1409     1445      +36     
- Misses         73       89      +16     

Files	Coverage Δ
custom_components/mail_and_packages/config_flow.py	98.31% <25.00%> (-1.69%)	⬇️

... and 2 files with indirect coverage changes