Add more descriptive errors to ncon

[VictorVanthilt]

The invalid NCON network error did not provide the user with information on why the network did not comply with the ncon style, while this information was available during the checking procedure.

This PR provides the user more information on why their network did not conform to the ncon style.

[codecov]

Codecov Report

❌ Patch coverage is 92.30769% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 82.05%. Comparing base (455a69f) to head (b1009fd).
⚠️ Report is 50 commits behind head on master.

Files with missing lines	Patch %	Lines
src/indexnotation/ncontree.jl	91.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #216      +/-   ##
==========================================
- Coverage   83.06%   82.05%   -1.02%     
==========================================
  Files          25       30       +5     
  Lines        2150     2564     +414     
==========================================
+ Hits         1786     2104     +318     
- Misses        364      460      +96     

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[Jutho]

Looks good to me; I'll merge and include it in the 5.3 release.