fix: validate_schemas names its jsonschema install

Without jsonschema the documented command stopped on an ImportError
traceback. Run as a command it now prints the install pipeline.py names
for the same step, with the format-nongpl extra, and exits 1. An import
from the tests still raises ImportError, which they skip on.

scripts/validate_schemas.py

@@ -14,7 +14,15 @@ from pathlib import Path, PurePosixPath
 import yaml
 
 from common import source_yaml_paths, yaml_load
-from jsonschema import Draft202012Validator, FormatChecker
+
+try:
+    from jsonschema import Draft202012Validator, FormatChecker
+except ImportError:
+    # Tests import this module and skip on the ImportError, the command
+    # names the install pipeline.py names for the same step.
+    if __name__ != "__main__":
+        raise
+    sys.exit('Error: jsonschema required (pip install "jsonschema[format-nongpl]")')
 
 ROOT = Path(__file__).resolve().parent.parent
 SCHEMAS = ROOT / "schemas"

tests/test_schema_semantic_gate.py

@@ -120,5 +120,31 @@ class DataRootIsKeyedBySystem(unittest.TestCase):
         self.assertTrue(self._errors({}))
 
 
+class WithoutJsonschema(unittest.TestCase):
+    """The documented command stopped on a bare ImportError traceback that
+    named neither the install nor the format-nongpl extra."""
+
+    def test_the_command_names_the_install(self):
+        import os
+        import subprocess
+
+        scratch = REPO_ROOT / "tmp" / "tests"
+        scratch.mkdir(parents=True, exist_ok=True)
+        with tempfile.TemporaryDirectory(dir=scratch) as directory:
+            stub = Path(directory) / "jsonschema"
+            stub.mkdir()
+            (stub / "__init__.py").write_text(
+                'raise ImportError("No module named jsonschema")\n', encoding="utf-8"
+            )
+            env = {**os.environ, "PYTHONPATH": directory}
+            proc = subprocess.run(
+                [sys.executable, "scripts/validate_schemas.py", "--source-only"],
+                capture_output=True, text=True, check=False, cwd=REPO_ROOT, env=env,
+            )
+        self.assertEqual(proc.returncode, 1)
+        self.assertNotIn("Traceback", proc.stderr)
+        self.assertIn('pip install "jsonschema[format-nongpl]"', proc.stderr)
+
+
 if __name__ == "__main__":
     unittest.main()