Fix unclear configure import error

[AndrewAsseily]

Issue #, if available: CLI-4867

Description of changes:
This enhancement addresses the issue reported in #7758, where users were encountering unclear error messages when attempting to import CSV credentials.
This change improves the documentation and error messaging for the aws configure import --csv
command when the file:// prefix is missing from file paths.

Testing:

1. Ran Unit tests
2. Built and reviewed documentation changes
3. Manually tested scenarios:

   1. Missing file:// prefix:

      ./bin/aws configure import --csv test-creds.csv
      

      Result: Shows error message

      You may be passing a file to import without the 'file://' prefix. To import a CSV file, use --csv file://path/to/file.csv

   2. Correct file path format:

      ./bin/aws configure import --csv file://test-creds.csv
      

      Result: Shows success message

      Successfully imported 1 profile(s)

   3. Inline CSV data:

      ./bin/aws configure import --csv "User Name,Access Key ID,Secret Access Key
      my-user,*************,inlinekeyvalue"
      

      Result: Shows success message

      Successfully imported 1 profile(s)

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this contribution, under the terms of your choice.

[kdaily]

Overall looks good! One note on the error that we can raise.

[kdaily]

Only a couple of minor changes, but this change should improve customer experience in adding creds from CSV files!