Fix 'raise' without Exception

[dweindl]

and fix error message and simplify function. Fixes #633

[codecov-commenter]

Codecov Report

Merging #696 (ccbd0c1) into develop (c58aaab) will increase coverage by 0.03%.
The diff coverage is 33.33%.

@@             Coverage Diff             @@
##           develop     #696      +/-   ##
===========================================
+ Coverage    90.08%   90.11%   +0.03%     
===========================================
  Files           96       96              
  Lines         6513     6513              
===========================================
+ Hits          5867     5869       +2     
+ Misses         646      644       -2     

Impacted Files	Coverage Δ
pypesto/visualize/clust_color.py	91.25% <33.33%> (ø)
pypesto/sample/geweke_test.py	94.36% <0.00%> (+2.81%)	⬆️

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[dweindl]

ping @jvanhoefer