Fix: Move exceptions into Exception namespace

[localheinz]

This PR

• moves exceptions into the Exception namespace

[codecov]

Codecov Report

Merging #26 into master will not change coverage by %.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master      #26   +/-   ##
=========================================
  Coverage     61.85%   61.85%           
  Complexity       80       80           
=========================================
  Files             8        8           
  Lines           194      194           
=========================================
  Hits            120      120           
  Misses           74       74           

Impacted Files	Coverage Δ	Complexity Δ
src/Definition/Definitions.php	92.85% <ø> (ø)	13.00 <0.00> (ø)
src/Exception/EntityDefinitionUnavailable.php	100.00% <ø> (ø)	2.00 <0.00> (?)
src/Exception/InvalidDefinition.php	100.00% <ø> (ø)	1.00 <0.00> (?)
src/Exception/InvalidDirectory.php	100.00% <ø> (ø)	1.00 <0.00> (?)
src/FixtureFactory.php	93.82% <100.00%> (ø)	36.00 <0.00> (ø)

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