Update error for missing koenig styles

[royalfig]

When recently helping a customer update to v5, Jon got hit with this error message, which he found confusing for him and the customer. In particular, the bit about affected files: styles. The difficulty here is that the error message is not about a specific file — but about missing files.

This commit updates the error message to be more explicit: Missing styles or stylesheets

[naz]

@royalfig did you check how these changes appear in Ghost Admin?