Poor error message when mixing list and key-value syntax #1916

Given the following input:

linter:
  rules:
    close_sinks: false
    - empty_statements 

The message produced is

Expected a key while parsing a block mapping.

While the message is technically correct, it doesn't help the author of the file figure out how to fix the issue as much as it could. A user suggested something like

You can’t mix list and key-value syntax in the same object.

Please improve the error message for this situation.