More detailed parser errors

[blegat]

This improves the errors obtained when parsing an incorrect string.

[codecov-io]

Codecov Report

Merging #825 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #825      +/-   ##
==========================================
+ Coverage   93.49%   93.51%   +0.02%     
==========================================
  Files          70       71       +1     
  Lines        7610     7653      +43     
==========================================
+ Hits         7115     7157      +42     
- Misses        495      496       +1

Impacted Files	Coverage Δ
src/Utilities/parser.jl	97.43% <100%> (+0.11%)	⬆️
src/Bridges/Variable/free.jl	87.87% <0%> (ø)
src/Bridges/bridge_optimizer.jl	90.67% <0%> (+1.02%)	⬆️

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[mlubin]

Thanks! I was pretty lazy in writing the parser given that it's for internal use only.