Correction for handling missing parquet inputs

[gavinevans]

Addresses metoppv#2135

Description
Add handling for missing parquet inputs when training. Adds CLI acceptance tests to demonstrate behaviour.

Testing:

• Ran tests and they passed OK
• Added new tests for the new feature(s)

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (mobt_877_implement_qrf@0a3e0f0). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
...oad_and_train_quantile_regression_random_forest.py	0.00%	2 Missing ⚠️
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Additional details and impacted files

@@                    Coverage Diff                    @@
##             mobt_877_implement_qrf      #19   +/-   ##
=========================================================
  Coverage                          ?   95.69%           
=========================================================
  Files                             ?      142           
  Lines                             ?    14162           
  Branches                          ?        0           
=========================================================
  Hits                              ?    13552           
  Misses                            ?      610           
  Partials                          ?        0           

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