Improved exceptions and tests for them

[peterfox]

Changes In Code

Better exceptions

Issue ticket number / Business Case

Some exceptions where confusing.

Checklist before requesting a review

• I have written PHPUnit tests.
• I have updated the documentation and opened a pull request within
  the feature flags documentation repo.
• I have checked code styles, PHPStan etc. pass.
• I have provided an issue/business case.
• I have added the enhancement label for a new feature.

[codecov]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (39baec4) 83.09% compared to head (ff3ea2b) 82.57%.
Report is 1 commits behind head on main.

Files	Patch %	Lines
src/Exceptions/FileNotFound.php	50.00%	2 Missing ⚠️
src/Exceptions/GatewayConfigurationMissing.php	50.00%	2 Missing ⚠️
src/Exceptions/UnableToLoadFlags.php	50.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main      #59      +/-   ##
============================================
- Coverage     83.09%   82.57%   -0.53%     
- Complexity      202      208       +6     
============================================
  Files            31       34       +3     
  Lines           556      568      +12     
============================================
+ Hits            462      469       +7     
- Misses           94       99       +5     

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