Improve error messages

[jeffreylovitz]

Report file name and line number in schema exceptions.

[codecov]

Codecov Report

Merging #57 (8ed9c54) into master (b6b32a4) will decrease coverage by 0.36%.
The diff coverage is 57.14%.

@@            Coverage Diff             @@
##           master      #57      +/-   ##
==========================================
- Coverage   88.82%   88.46%   -0.37%     
==========================================
  Files           9        9              
  Lines         564      572       +8     
==========================================
+ Hits          501      506       +5     
- Misses         63       66       +3     

Impacted Files	Coverage Δ
redisgraph_bulk_loader/entity_file.py	88.00% <20.00%> (-0.44%)	⬇️
redisgraph_bulk_loader/relation_type.py	87.14% <60.00%> (-2.41%)	⬇️
redisgraph_bulk_loader/label.py	91.80% <100.00%> (+0.42%)	⬆️

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