Adjust error message to show that the capacity reservation does not exist

[hgreebe]

Description of changes

• Adjust error message to show that the capacity reservation does not exist.
• Orginal error can not be thrown here because then it causes a failure during cluster update.
• This change only catches the error during cluster creation validation

Tests

• Tested that the appropriate error message occurs during cluster creating when a capacity reservation is specified but not the instance type
• Tested that the appropriate error message occurs during cluster creation when neither a capacity reservation or instance type is specified.
• Ran full set of integration tests.

References

• Related to this change: [Release-3.13] Address cluster update failure when old capacity reservation has been deleted #6883

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[gmarciani]

The logici around exception raising should not be based on error messages, as it is a fragile approach. Error messages are meant to change over time.
The logic should be based on something more stable, such as exception types.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 90.20%. Comparing base (960668b) to head (6672d3a).
Report is 32 commits behind head on release-3.13.

Files with missing lines	Patch %	Lines
cli/src/pcluster/models/cluster.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff                @@
##           release-3.13    #6887      +/-   ##
================================================
- Coverage         90.21%   90.20%   -0.01%     
================================================
  Files               181      181              
  Lines             16205    16211       +6     
================================================
+ Hits              14619    14623       +4     
- Misses             1586     1588       +2     

Flag	Coverage Δ
unittests	90.20% <0.00%> (-0.01%)	⬇️

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