[projmgr] Improve regions file existence check

[spcaipers-arm]

Apply to regions file the logic already in place for user files: skip throwing an error when the file does not exist if it's expected to be generated by an execute node.
Address Open-CMSIS-Pack/cmsis-toolbox#245

[github-actions]

Test Results

    3 files     18 suites   10m 7s ⏱️
  381 tests   381 ✅ 0 💤 0 ❌
1 143 runs  1 143 ✅ 0 💤 0 ❌

Results for commit fe01501.

[codecov]

Codecov Report

Attention: Patch coverage is 54.54545% with 5 lines in your changes missing coverage. Please review.

Project coverage is 65.16%. Comparing base (0fcca05) to head (fe01501).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
tools/projmgr/test/src/ProjMgrUnitTests.cpp	20.00%	0 Missing and 4 partials ⚠️
tools/projmgr/test/src/ProjMgrWorkerUnitTests.cpp	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1921   +/-   ##
=======================================
  Coverage   65.16%   65.16%           
=======================================
  Files         173      173           
  Lines       34850    34848    -2     
  Branches    20227    20227           
=======================================
+ Hits        22709    22710    +1     
+ Misses       7960     7959    -1     
+ Partials     4181     4179    -2     

Flag	Coverage Δ
projmgr-cov	81.89% <54.54%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
tools/projmgr/src/ProjMgrWorker.cpp	88.13% <100.00%> (+0.07%)	⬆️
tools/projmgr/test/src/ProjMgrWorkerUnitTests.cpp	65.53% <66.66%> (+0.15%)	⬆️
tools/projmgr/test/src/ProjMgrUnitTests.cpp	80.94% <20.00%> (-0.06%)	⬇️