catch + print errors manually

[SimonDanisch]

Julia does seem to have problems with errors setindex(obs, val) - it might stack overflow or just silently freeze. A try catch and manual error display seems to solve this

[codecov-io]

Codecov Report

Merging #26 into master will decrease coverage by 1.67%.
The diff coverage is 60%.

@@            Coverage Diff             @@
##           master      #26      +/-   ##
==========================================
- Coverage   98.19%   96.52%   -1.68%     
==========================================
  Files           5        5              
  Lines         111      115       +4     
==========================================
+ Hits          109      111       +2     
- Misses          2        4       +2

Impacted Files	Coverage Δ
src/Observables.jl	92.85% <60%> (-3.3%)	⬇️

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