metno nwp prepare fails when input file does not exists

[TAlonglong]

There was an assumption that some input file exist. If it does not some eccodes function raises and the preparation fails.

Add check to avoid this.

[codecov]

Codecov Report

Merging #45 (a103dc1) into main (a744cb8) will decrease coverage by 0.24%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main      #45      +/-   ##
==========================================
- Coverage   53.94%   53.70%   -0.25%     
==========================================
  Files          14       14              
  Lines        1774     1782       +8     
  Branches      213      238      +25     
==========================================
  Hits          957      957              
- Misses        799      807       +8     
  Partials       18       18              

Flag	Coverage Δ
unittests	53.70% <0.00%> (-0.25%)	⬇️

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Impacted Files	Coverage Δ
nwcsafpps_runner/metno_update_nwp.py	0.00% <0.00%> (ø)

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