fix: improve extension startup error messages with command details

[wonrax]

Before:

     Running `target/debug/goose`
Failed to start extension: git, "No such file or directory (os error 2)"
Please check extension configuration for git.

The user might mistake this to mean that the git command is not installed and available in their $PATH. I fixed the error message to be more descriptive.

After:

     Running `target/debug/goose`
Failed to start extension `git`: Could not run extension command (`uvxx mcp-server-git`): No such file or directory (os error 2)
Please check extension configuration for git.

[wonrax]

@michaelneale I don't know who else to mention, but can you check this? If this can break the user API feel free to close it.