Improve error messages

[ThijsvdLaar]

This PR fixes #27 , #70 , #146 , #194

[codecov-commenter]

Codecov Report

Merging #204 (c1a905a) into master (a3788e1) will increase coverage by 0.12%.
The diff coverage is 86.66%.

@@            Coverage Diff             @@
##           master     #204      +/-   ##
==========================================
+ Coverage   87.14%   87.27%   +0.12%     
==========================================
  Files          97       97              
  Lines        4614     4645      +31     
==========================================
+ Hits         4021     4054      +33     
+ Misses        593      591       -2     

Impacted Files	Coverage Δ
src/distribution.jl	73.33% <0.00%> (-2.01%)	⬇️
src/engines/julia/update_rules/gaussian_moments.jl	95.55% <ø> (+3.55%)	⬆️
src/message_passing.jl	82.67% <60.00%> (-1.33%)	⬇️
src/algorithms/inference_algorithm.jl	90.47% <75.00%> (-5.45%)	⬇️
src/algorithms/expectation_propagation.jl	95.38% <100.00%> (+2.63%)	⬆️
src/algorithms/joint_marginals.jl	96.84% <100.00%> (+1.89%)	⬆️
src/algorithms/naive_variational_bayes.jl	96.82% <100.00%> (+2.79%)	⬆️
src/algorithms/posterior_factorization.jl	96.55% <100.00%> (+0.22%)	⬆️
src/algorithms/structured_variational_bayes.jl	96.38% <100.00%> (+2.13%)	⬆️
src/algorithms/sum_product.jl	96.47% <100.00%> (+0.96%)	⬆️
... and 8 more

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