fix: check file-writer construction when writing envelope to path

[supervacuus]

fixes #1035.

• Fixed two other missing path checks and an incorrect file-writer loop on Windows.
• Added convenience powershell scripts for local test runs on Windows.
• Clarified platform support of sentry_handle_exception() as requested here: Implement sentry_handle_exception for macOS and Linux when using crashpad #1032 (comment)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.78%. Comparing base (a762b23) to head (e9b664a).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1036      +/-   ##
==========================================
+ Coverage   83.75%   83.78%   +0.02%     
==========================================
  Files          53       53              
  Lines        5510     5512       +2     
  Branches     1197     1198       +1     
==========================================
+ Hits         4615     4618       +3     
+ Misses        782      781       -1     
  Partials      113      113