Improve error message on "failed to spawn" where the Python interpreter is missing

Summary

e.g., as reported in https://github.com/astral-sh/uv/issues/13196

The error message when the Python interpreter cannot be found by an entry point is

error: Failed to spawn: `jupyter` Caused by: No such file or directory (os error 2)

We can read the entry point and provide a better error message in this case.

Related #13989

Example

error: Failed to spawn: `jupyter` Caused by: No such file or directory (os error 2)

hint: `jupyter` uses a Python interpreter at `path` which no longer exists