Fix unhelpful error message for non-existing inputs to veros run

[dionhaefner]

No description provided.

[codecov]

Codecov Report

Merging #259 (698c22f) into master (8da84b1) will not change coverage.
The diff coverage is 50.00%.

@@           Coverage Diff           @@
##           master     #259   +/-   ##
=======================================
  Coverage   82.34%   82.34%           
=======================================
  Files          84       84           
  Lines        7257     7257           
  Branches      875      875           
=======================================
  Hits         5976     5976           
  Misses        984      984           
  Partials      297      297           

Impacted Files	Coverage Δ
veros/cli/veros_run.py	42.64% <50.00%> (ø)

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