Fix #499

[elbaraim]

This is a bug that I found while performing sampling. Also reported in #499
In the matlab PESTO, if a failed log posterior evaluation happened, i.e., returning NaN, the log posterior value would be set to -Inf.
This exception was missing in the current pyPESTO implementation, now added.

[elbaraim]

something went wrong with merging, I will fix this now, sorry!

[codecov-io]

Codecov Report

Merging #533 (6ad4d5d) into develop (1aefa58) will increase coverage by 0.02%.
The diff coverage is 50.00%.

@@             Coverage Diff             @@
##           develop     #533      +/-   ##
===========================================
+ Coverage    90.76%   90.79%   +0.02%     
===========================================
  Files           65       65              
  Lines         4212     4214       +2     
===========================================
+ Hits          3823     3826       +3     
+ Misses         389      388       -1     

Impacted Files	Coverage Δ
pypesto/sample/metropolis.py	97.36% <50.00%> (-1.29%)	⬇️
pypesto/engine/multi_process.py	92.30% <0.00%> (+7.69%)	⬆️

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[elbaraim]

something went wrong with merging, I will fix this now, sorry!

now should be fine to review 👍