feat: re-export apify-client errors from apify.errors

[vdusek]

Description

Adds an apify.errors module that re-exports the apify_client error hierarchy, so SDK users have a single import location for every error the SDK can surface. The SDK raises these client exceptions as-is and does not wrap them.

Actor.call and Actor.call_task keep returning the finished run regardless of status. Check run.status to react to a failed run.

A new "Error handling" concept page maps every layer of exceptions an Actor can hit:

• API client errors (ApifyApiError and its status-code subclasses).
• The standard RuntimeError, ValueError, TypeError, and ConnectionError the SDK raises for misuse and invalid input.
• Run-status checking after Actor.call and Actor.call_task.
• The pay-per-event charge limit, which is a return value rather than an exception.
• Short mention of errors while crawling and links Crawlee docs.

Issue

Closes #988

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.61%. Comparing base (482e725) to head (773a242).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #990   +/-   ##
=======================================
  Coverage   91.61%   91.61%           
=======================================
  Files          49       50    +1     
  Lines        3171     3174    +3     
=======================================
+ Hits         2905     2908    +3     
  Misses        266      266           

Flag	Coverage Δ
e2e	35.66% <0.00%> (-0.04%)	⬇️
integration	56.36% <0.00%> (-0.06%)	⬇️
unit	80.52% <100.00%> (+0.01%)	⬆️

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