Reworded content load exception message

[Doprez]

PR Details

The previous error did not explain the problem very clearly, this attempts to help with readability.

Related Issue

https://github.com/stride3d/stride/issues/2178

Motivation and Context

Trying to make things easier to read for newcomers is a benefit.

Types of changes

• [x] Docs change / refactoring / dependency upgrade
• [ ] 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

• [ ] My change requires a change to the documentation.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.
• [x] I have built and run the editor to try this change out.