Enhancement: Throw InvalidCount exception

[localheinz]

This PR

• throws an InvalidCount exception when an invalid number of entities is requested

Follows #87.

[codecov]

Codecov Report

Merging #105 into master will increase coverage by 0.74%.
The diff coverage is 94.73%.

@@             Coverage Diff              @@
##             master     #105      +/-   ##
============================================
+ Coverage     93.75%   94.49%   +0.74%     
- Complexity       76       77       +1     
============================================
  Files             9       10       +1     
  Lines           208      218      +10     
============================================
+ Hits            195      206      +11     
+ Misses           13       12       -1     

Impacted Files	Coverage Δ	Complexity Δ
src/FixtureFactory.php	91.86% <88.88%> (+0.20%)	44.00 <5.00> (ø)
src/Exception/InvalidCount.php	100.00% <100.00%> (ø)	1.00 <1.00> (?)
src/FieldDefinition.php	100.00% <100.00%> (+4.34%)	6.00 <2.00> (ø)

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