Show "Not parsing changed paths due to missing or broken rill.yaml" warning in the Rill Developer UI

[ericpgreen2]

When a rill.yaml file cannot be parsed, the following warning is emitted in the terminal:

We should show a similar warning in the UI.

In these cases, the editor appears broken. Below, see that the result table ("Hello") does not reflect the code in the editor ("Hello world"):

[ericpgreen2]

@jkhwu could you please propose a design & place in the UI to put a persistent warning message like: "The rill.yaml file cannot be parsed. Please fix before proceeding."