indicate cause of failure for `RenderPipelineCache::get()`

## What problem does this solve or what need does it fill?

Currently, a user can define a shader handle, use it in a `RenderPipelineDescriptor`, and pass that descriptor to `RenderPipelineCache::queue()` without ever associating the handle with an actual `Shader`. The pipeline descriptor remains queued indefinitely with no warnings emitted to the user.

## What solution would you like?

Change the return type of `RenderPipelineCache::get()` to a `Result` whose `Err` variant indicates the reason that the descriptor is still queued.

## What alternative(s) have you considered?

- Track the number of calls to `RenderPipelineCache::process_queue()` since the descriptor was queued and emit a warning after some threshold.