Raise error when trying to visualize tree with enable_tree_vis=false

[BoZenKhaa]

Addresses #97

[codecov]

Codecov Report

Base: 85.83% // Head: 86.12% // Increases project coverage by +0.28% 🎉

Coverage data is based on head (f5c50d7) compared to base (16042d9).
Patch coverage: 80.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #98      +/-   ##
==========================================
+ Coverage   85.83%   86.12%   +0.28%     
==========================================
  Files          10       10              
  Lines         473      490      +17     
==========================================
+ Hits          406      422      +16     
- Misses         67       68       +1     

Impacted Files	Coverage Δ
src/vanilla.jl	88.51% <75.00%> (+0.40%)	⬆️
src/visualization.jl	95.19% <100.00%> (-0.65%)	⬇️
src/dpw.jl	94.82% <0.00%> (+0.18%)	⬆️

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[zsunberg]

Thanks for the contribution! This is a good change to make, but please see my comments in the code.

[zsunberg]

Oops I misunderstood something in my comment. It looks good if you change the error message like I suggested!

[BoZenKhaa]

Ok, thanks for the feedback, done!

[zsunberg]

Thanks!