Use an exception catching to avoid a stack trace

[Gu1nness]

When exclude files are used, this can trigger a stack trace if the file
does not exist. Instead exit nicely without the stacktrace.

[codecov-commenter]

Codecov Report

Merging #5957 (c12fc71) into 1.1-maint (3044a34) will decrease coverage by 0.03%.
The diff coverage is 66.66%.

@@              Coverage Diff              @@
##           1.1-maint    #5957      +/-   ##
=============================================
- Coverage      79.78%   79.75%   -0.04%     
=============================================
  Files             27       27              
  Lines          10460    10464       +4     
  Branches        1841     1841              
=============================================
  Hits            8346     8346              
- Misses          1590     1594       +4     
  Partials         524      524              

Impacted Files	Coverage Δ
src/borg/patterns.py	91.37% <66.66%> (-0.86%)	⬇️
src/borg/archiver.py	80.97% <0.00%> (-0.15%)	⬇️
src/borg/helpers.py	87.84% <0.00%> (-0.08%)	⬇️
src/borg/remote.py	80.12% <0.00%> (ø)
src/borg/archive.py	81.24% <0.00%> (+0.14%)	⬆️

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[ThomasWaldmann]

Thanks!