feat: add error_if_* operators

[bvdmitri]

This PR adds error_if and error_if_not operator that might be useful for some checks (e.g. check NaN in free energy computation in ReactiveMP.jl)

[codecov-commenter]

Codecov Report

Merging #26 (25f6f8d) into master (fe134e9) will increase coverage by 0.20%.
The diff coverage is 94.73%.

@@            Coverage Diff             @@
##           master      #26      +/-   ##
==========================================
+ Coverage   77.88%   78.08%   +0.20%     
==========================================
  Files         115      117       +2     
  Lines        3531     3569      +38     
==========================================
+ Hits         2750     2787      +37     
- Misses        781      782       +1     

Impacted Files	Coverage Δ
src/Rocket.jl	100.00% <ø> (ø)
src/operators/error_if.jl	94.59% <94.59%> (ø)
src/operators/error_if_not.jl	100.00% <100.00%> (ø)
src/actor/test.jl	80.36% <0.00%> (+0.36%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update fe134e9...25f6f8d. Read the comment docs.