Type-cast inputs for general Error

[CasperWA]

Fixes #281

I had an issue, where detail wasn't passed as a string, and it failed to produce a response.
This fixes it, by explicitly type-casting all values needed for the most general exception creation of an OptimadeError

[codecov]

Codecov Report

Merging #280 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #280      +/-   ##
==========================================
+ Coverage   89.93%   89.94%   +0.01%     
==========================================
  Files          54       54              
  Lines        2245     2248       +3     
==========================================
+ Hits         2019     2022       +3     
  Misses        226      226              

Flag	Coverage Δ
#unittests	89.94% <100.00%> (+0.01%)	⬆️

Impacted Files	Coverage Δ
optimade/server/exception_handlers.py	80.30% <100.00%> (+0.93%)	⬆️

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[ml-evs]

Looks fine to me, I can't foresee any possible adverse effects of this fix.