Fix config file warnings when file is missing

[ml-evs]

Closes #930.

If config loading provides any warnings, then show them.

[codecov]

Codecov Report

Merging #931 (f43983c) into master (4adbaa6) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #931   +/-   ##
=======================================
  Coverage   92.78%   92.78%           
=======================================
  Files          67       67           
  Lines        3783     3783           
=======================================
  Hits         3510     3510           
  Misses        273      273           

Flag	Coverage Δ
project	92.78% <100.00%> (ø)
validator	92.78% <100.00%> (ø)

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Impacted Files	Coverage Δ
optimade/server/main.py	94.33% <100.00%> (ø)
optimade/server/main_index.py	94.73% <100.00%> (ø)

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[JPBergsma]

I do not have any remarks about the changes you made.
There is however quite some duplicate code in these files and I am wondering whether it would be possible to reduce this somehow.

[ml-evs]

There is however quite some duplicate code in these files and I am wondering whether it would be possible to reduce this somehow.

The main/main_index servers are meant to be individually "standalone" so I'm not sure if its worth wrapping this logging into a function.