[bugfix] Fix ReFrame crash when trying to retrieve source code context during errors and treat Unicode errors gracefully

[vkarak]

Closes #3631.

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.25%. Comparing base (62babc6) to head (5788b61).

Files with missing lines	Patch %	Lines
reframe/frontend/printer.py	0.00%	2 Missing ⚠️
reframe/core/exceptions.py	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3632      +/-   ##
==========================================
- Coverage   91.25%   91.25%   -0.01%     
==========================================
  Files          62       62              
  Lines       13587    13591       +4     
==========================================
+ Hits        12399    12402       +3     
- Misses       1188     1189       +1     

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