Enhancement: Throw InvalidFieldNames exception

[localheinz]

This PR

• throws a FieldsNotFound exception instead of a generic Exception when fields are referenced that are not found in the corresponding entity

[codecov]

Codecov Report

Merging #87 into master will increase coverage by 0.27%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master      #87      +/-   ##
============================================
+ Coverage     93.50%   93.77%   +0.27%     
- Complexity       75       77       +2     
============================================
  Files             8        9       +1     
  Lines           200      209       +9     
============================================
+ Hits            187      196       +9     
  Misses           13       13              

Impacted Files	Coverage Δ	Complexity Δ
src/EntityDef.php	95.45% <100.00%> (-0.11%)	13.00 <0.00> (ø)
src/Exception/InvalidFieldNames.php	100.00% <100.00%> (ø)	2.00 <2.00> (?)
src/FixtureFactory.php	90.80% <100.00%> (+0.10%)	36.00 <0.00> (ø)

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