Send all user messages to STDERR when writing to credentials file

[epierce]

Description

Running $(gimme-aws-creds) would fail when writing to a credentials file because notices to the user were written to STDOUT instead of STDERR. This change sends all messages to the user using STDERR.

Related Issue

#156

How Has This Been Tested?

tested on macOS Ventura with Python 3.11

Types of changes

• [x ] Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x ] My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• [ x] I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• [ x] All new and existing tests passed.