DEVEP-2794: Don't change remote image paths if download fails

[macdonst]

Description

Catches invalid image download results

Related Issue

DEVEP-2794

Motivation and Context

Some remote images linked via the docs may not be accessible to our build project so instead of having the downloader have an invalid image file it catches these errors and leaves the markdown unchanged.

How Has This Been Tested?

Added test cases
Ran markdown cleaner against a failing repo

Screenshots (if appropriate):

n/a

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:

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