Add error message when no params selected

[jescalada]

Currently, when no metrics are selected in the Metrics app, we get a error message (although this might be because the backend is erroring out, and the error is caught in the frontend):

This isn't the case for the Params app, where there is no error message displayed, and furthermore, the experiment selection header's state does not update when searching. We'd like the behaviour of the Params to match the Metrics.

[jescalada]

Note: Now we want to remove the error message when no metrics are selected in the Metrics app. This may cause issues with re-rendering selected experiment status, so some extra testing will be required.