MAINT: improve error message when meson isn't found

[rgommers]

This is a follow-up to gh-495.

There are several ways that meson may go missing:

• a non-isolated build and meson isn't installed
• meson executable is specified in pyproject.toml and git submodule is containing it is not initialized (reported as resulting in a confusing error in https://github.com/numpy/numpy/issues/26397)
• the MESON environmnent variable is used by the user and is misspelled

The git submodule case was ending with:

meson-python: error: Could not find meson version 0.63.3 or newer, found .

and after this ends with:

meson-python: error: Could not find specified meson: vendored-meson/meson/meson.py

If the executable is missing, the build ended with a very long traceback. Changing FileNotFoundError to ConfigError elides the traceback and clearly reports that the executable wasn't found. Easy to verify with any package with:

$ MESON=nonsense python -m build -wnx
* Building wheel...

meson-python: error: meson executable `nonsense` not found

ERROR Backend subproccess exited when trying to invoke build_wheel

Closes https://github.com/numpy/numpy/issues/26397

[rgommers]

Thanks @dnicolodi. Adopted your suggestions for error messages.

In that I also check the return code of meson --version and raise an error with what was printed on the standard error if the return code is not zero.

If you have that already, do you want to put that on top of this PR (either push a commit or do in a follow-up PR)?

[dnicolodi]

Sure, I will do that later today.