Improve formatting of overwrite error

[nick-stroud]

Previous cli output:

$ ./ghpc create  examples/my_blueprint.yaml 
2022/04/21 14:08:15 Failed to overwrite existing blueprint. Use the -w command line argument to enable overwrite. If overwrite is already enabled then this may be because you are attempting to remove a resource group, which is not supported : The directory already exists: my_blueprint

Updated output

$ ./ghpc create  examples/my_blueprint.yaml 

Failed to overwrite existing blueprint.

Use the -w command line argument to enable overwrite.
If overwrite is already enabled then this may be because you are attempting to remove a resource group, which is not supported.
original error: The directory already exists: my_blueprint

Submission Checklist

• Have you installed and run this change against pre-commit? pre-commit install
• Are all tests passing? make tests
• If applicable, have you written additional unit tests to cover this
  change?
• Is unit test coverage still above 80%?
• Have you updated any application documentation such as READMEs and user
  guides?
• Have you followed the guidelines in our Contributing document?