[workload] Machine readable outputs should only output JSON

Is your feature request related to a problem? Please describe.

Commands like dotnet workload update --print-rollback and dotnet workload list --machine-readable are very helpful, but they should print only JSON to stdout.

Right now I run dotnet workload list --machine-readable and this is what gets printed to stdout:

Failed to update the advertising manifest microsoft.net.sdk.tvos: Unable to load the service index for source https://REDACTED/index.json..
Failed to update the advertising manifest microsoft.net.sdk.android: Unable to load the service index for source https://REDACTED/index.json..
Failed to update the advertising manifest microsoft.net.sdk.maui: Unable to load the service index for source https://REDACTED/index.json..
Failed to update the advertising manifest microsoft.net.workload.emscripten: Unable to load the service index for source https://REDACTED/index.json..
Failed to update the advertising manifest microsoft.net.sdk.macos: Unable to load the service index for source https://REDACTED/index.json..
Failed to update the advertising manifest microsoft.net.sdk.maccatalyst: Unable to load the service index for source https://REDACTED/index.json..
Failed to update the advertising manifest microsoft.net.sdk.ios: Unable to load the service index for source https://REDACTED/index.json..
Failed to update the advertising manifest microsoft.net.workload.mono.toolchain: Unable to load the service index for source https://REDACTED/index.json..
==workloadListJsonOutputStart==
{"installed":["macos","ios"],"updateAvailable":[{"existingManifestVersion":"12.0.101-preview.10.249","availableUpdateManifestVersion":"12.0.101-preview.10.251","description":".NET SDK Workload for building macOS applications.","workloadId":"macos"},{"existingManifestVersion":"15.0.101-preview.9.31","availableUpdateManifestVersion":"15.0.101-preview.10.251","description":".NET SDK Workload for building iOS applications.","workloadId":"ios"}]}
==workloadListJsonOutputEnd==

I have to process this and it complicates my tooling.

Describe the solution you’d like

Commands like rg, lsblk, losetup, etc that offer JSON output modes only print JSON to stdout. Everything else goes to stderr. dotnet workload commands that are intended to offer machine-readable output would be most useful if they functioned similarly.

This opens up much easier shell scripting workflows. In powershell you could pipe to ConvertFrom-Json without any other ceremony. In other shells you could pipe to jq.

Additional context

I would imagine this would break compatibility with tools used to the current output, so unfortunately it would require another option, like --really-machine-readable.

[sandyarmstrong]

We likely wouldn’t need those if that output were hidden by default (and would only include the indicators when verbosity level is high).

Could also just output the manifest errors and other verbose items to stderr instead of stdout.

[baronfel]

Summary for anyone that picks this up:

• We should emit the verbose “failed to …” output to stderr
• We should remove the boundary markers