Print missing filename

[dnerini]

when looking into #426, I noticed that the the pysteps.io.archive.find_by_date function wrongly logs None instead of the wrong filename. This PR fixes it.

[codecov]

Codecov Report

Attention: Patch coverage is 16.66667% with 5 lines in your changes missing coverage. Please review.

Project coverage is 83.85%. Comparing base (e3eed9e) to head (77b4c1e).
Report is 1 commits behind head on master.

Files	Patch %	Lines
pysteps/io/archive.py	16.66%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #427      +/-   ##
==========================================
- Coverage   83.86%   83.85%   -0.01%     
==========================================
  Files         160      160              
  Lines       12781    12780       -1     
==========================================
- Hits        10719    10717       -2     
- Misses       2062     2063       +1     

Flag	Coverage Δ
unit_tests	83.85% <16.66%> (-0.01%)	⬇️

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