core: Service "Reload themes" (or a related core method) does not return an error if a theme is invalid

The problem

Originally reported for Frontend: https://github.com/home-assistant/frontend/issues/17610 Was advised to move the issue to the Core.

A detailed description is provided here: https://github.com/home-assistant/frontend/issues/17610. In short: – when a theme is invalid, a user must be informed (an error message in “Dev tools -> Services” or in a Log); – it should happen either when calling the “Reload themes” service or at HA startup.

Example of an invalid theme:

my_poor_theme:

i.e. w/o any variables.

What version of Home Assistant Core has the issue?

2023.8.0

What was the last working version of Home Assistant Core?

No response

What type of installation are you running?

Home Assistant Container

Integration causing the issue

No response

Link to integration documentation on our website

https://www.home-assistant.io/integrations/frontend/

Diagnostics information

No response

Example YAML snippet

No response

Anything in the logs that might be useful for us?

No response

Additional information

No response