EC2 facade: handle snapshot not found

[rbailey-godaddy]

Description

The logic originally used does not actually detect missing snapshots correctly, resulting in errors and stack traces instead of simple warnings as intended.

Fixes: #1433

Type of change

Select the relevant option(s):

• 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 not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works (optional)
• New and existing unit tests pass locally with my changes