Improve devcontainer error message when spec file is missing

## Problem
When a user deletes a repository containing a `.devcontainer.json` file, the workspace continues to show the devcontainer in the UI but throws a generic error when attempting to rebuild:

```
read devcontainer config: devcontainer read-configuration failed: exit status 1
```

## User Experience Issue
The current error message doesn't clearly indicate that the devcontainer spec file is missing from the expected path. Users are left confused about why their devcontainer is failing to build.

## Proposed Solution
- Replace the generic error with a more explicit message like: `no devcontainer spec found at $PATH`
- Consider surfacing a "remove" button in the UI for devcontainers that no longer exist
- At minimum, hint to users that they can manually remove the devcontainer configuration if it's no longer needed

## Context
This issue emerged from a user workflow where:
1. User had a workspace with a devcontainer
2. User deleted the repository containing the `.devcontainer.json`
3. Workspace UI still showed the devcontainer option
4. Attempting to rebuild resulted in an unclear error message

## Expected Outcome
Users should receive clear, actionable error messages when devcontainer specs are missing, and ideally have UI options to clean up stale configurations.