Report more error types in the UI

[ehmicky]

We surface plugin errors in the UI ("plugins error statuses"), i.e. not only in the build logs. We should also show the following specific error types in the UI:

• netlify.toml configuration error
• build.command failed
• internal error in Netlify Build not related to a specific plugin

One thing to consider: error statuses shown in the UI are currently plugin-related. The API endpoint plugin_runs asks for a plugin name, version and the event in which the error happened. This would not fit the errors above, so this would require either adding a new approach for those, or modifying the current approach, for both UI and the API.

Implementation note: in Netlify Build, the logic to retrieve the title, summary and text of error statuses (to send to the API) is not tied to plugins. It works for any error types (including the ones above), so we should re-use the same code we use for plugin error statuses.