fix: Fail early if private_key_path doesn't exist

[kronnk]

This pull request includes an important update to the get_private_key method in the target_snowflake/connector.py file. The changes enhance logging and error handling when reading the private key from a file.

Enhancements to logging and error handling:

• Added a debug log statement to indicate when the private key is being read from a file.
• Introduced a check to verify if the private key file exists before attempting to read it, raising a FileNotFoundError with a descriptive error message if the file is not found.

[edgarrmondragon]

Thanks!