Fix: Throw ClassMetadataNotFound exception

[localheinz]

This PR

• throws a ClassMetadataNotFound exception when class metadata could not be found for a referenced class name

Follows #1.

[codecov]

Codecov Report

Merging #126 into master will increase coverage by 0.05%.
The diff coverage is 66.66%.

@@             Coverage Diff              @@
##             master     #126      +/-   ##
============================================
+ Coverage     94.38%   94.44%   +0.05%     
- Complexity       68       71       +3     
============================================
  Files            11       12       +1     
  Lines           196      198       +2     
============================================
+ Hits            185      187       +2     
  Misses           11       11              

Impacted Files	Coverage Δ	Complexity Δ
src/FixtureFactory.php	91.50% <33.33%> (-0.08%)	39.00 <0.00> (+1.00)	⬇️
src/Exception/ClassMetadataNotFound.php	100.00% <100.00%> (ø)	2.00 <2.00> (?)

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