Return 404 instead of HTML for missing API methods.

[dennisoelkers]

Description

Motivation and Context

This PR changes the default catch-all method which is supposed to return the frontend bootstrapping HTML for deep-links to check if the missing URL is an API endpoint and return a 404 instead.

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

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

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have requested a documentation update.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.