Dev warning for bad component.observe

[Rich-Harris]

If someone does e.g. component.observe('foo.bar', ...) we should explain that you can only observe top-level properties when dev: true.

[Conduitry]

This is another case where we kind of need to be able to have separate dev mode and prod mode versions of a helper function. I've looked into that a little in #365 but perhaps that should be extracted into its own issue.