"could not read file as String: devtools_extensions/..." if pointing to different version of package without extension present

[mkorbel1]

This is splitting off a separate issue from #8912 from #8912 (comment) based on discussion in comment #8912 (comment).

If you point to a version of a package that has a devtool extension, then point to another version that does not have that extension present (i.e. built), you get an error about "could not read file".

If you point to a version of rohd which has a devtools extension present (e.g. the artifacts branch), and get it loaded up successfully, it then adds to the devtools_options.yaml file:

description: This file stores settings for Dart & Flutter DevTools.
documentation: https://docs.flutter.dev/tools/devtools/extensions#configure-extension-enablement-states
extensions:

• rohd: true
  If you then point to a version of rohd which does not have the devtools extension (e.g. the main branch), then when you open devtools, the rohd tab is still present but you get an error

could not read file as String: devtools_extensions/rohd_0.0.1/index.html

Is that the intended error message when an extension is not present?