`Download Previous Build` VMR build step should fail if no files are found/downloaded

The Stage 2 Fedora leg fails because it didn't download the source-built SDK. This leg fails during the build with the following error:
```
Custom SDK '/vmr/.dotnet/dotnet' does not exist or is not executable
```

The `Download Previous Build` leg succeeds, but it's clear that no files are actually downloaded:

```
Minimatch patterns: [**/Private.SourceBuilt.Artifacts.*.tar.gz,**/dotnet-sdk-*.tar.gz]
Filtered 0 files from the Minimatch filters supplied.
Downloaded 0.0 MB out of 0.0 MB (100%).
Downloaded 0.0 MB out of 0.0 MB (100%).

Download statistics:
Total Content: 0.0 MB
Physical Content Downloaded: 0.0 MB
Compression Saved: 0.0 MB
Local Caching Saved: 0.0 MB
Chunks Downloaded: 0
Nodes Downloaded: 0

Download completed.
ApplicationInsightsTelemetrySender correlated 2 events with X-TFS-Session 202b3c63-7961-4d57-9016-3626b8ae89a3
Downloading artifact finished.
Finishing: Download Previous Build
```

Rather than having the build leg fail, it would be more helpful to have the download leg fail with a "Not found" error if nothing is downloaded.

Even better, if the associated Stage 1 leg fails, the Stage 2 leg could be skipped entirely or output a message about the associated stage 1 leg failing.