feat(cdk/scrolling): Scroll functions silently fail if the viewport is not yet fully initialized

#### Feature Description

`cdk-virtual-scroll-viewport`'s `scrollTo...` functions (like `scrollToIndex`) silently fail when the viewport is not yet initialized. Technically the functions are doing nothing wrong from a code standpoint, but since the component is not yet fully initialized, it is empty, so trying to scroll will do nothing meaningful. Sending back an error (or at least logging a warning to the console) if an empty viewport is scrolled would be helpful so that developers know that something went wrong.

This would be mitigated if we had an async option that managed the timing of scroll commands. See [this feature request](https://github.com/angular/components/issues/23715).

#### Use Case

This would improve every use case of the virtual scroller.

Start at the cdk-virtual-scroll-viewport scrollTo... entry points and inspect how initialization and an empty viewport are represented. Confirm with maintainers whether the completed behavior should be an error or a console warning, then add focused coverage showing that scrolling before initialization no longer fails silently.

Written by the indexing model from the issue text.