MAINT: improve error message when meson isn't found

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 numpy#26397)
- 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 the 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
```

mesonpy/__init__.py

@@ -988,6 +988,8 @@ def _get_meson_command(
     # directly. For packages that vendor a forked Meson, the `meson.py` in the
     # root of the Meson repo can be used this way.
     if meson.endswith('.py'):
+        if not os.path.exists(meson):
+            raise ConfigError(f'Could not find the specified meson: "{meson}"')
         cmd = [sys.executable, meson]
     else:
         cmd = [meson]
@@ -997,7 +999,11 @@ def _get_meson_command(
     # but the corresponding meson command is not available in $PATH. Implement
     # a runtime check to verify that the build environment is setup correcly.
     required_version = _parse_version_string(version)
-    meson_version = subprocess.run(cmd + ['--version'], check=False, text=True, capture_output=True).stdout
+    try:
+        meson_version = subprocess.run(cmd + ['--version'], check=False, text=True, capture_output=True).stdout
+    except FileNotFoundError as err:
+        raise ConfigError(f'meson executable "{meson}" not found') from err
+
     if _parse_version_string(meson_version) < required_version:
         raise ConfigError(f'Could not find meson version {version} or newer, found {meson_version}.')