Use exit codes to indicate different kinds of error

It's helpful if we can surface the error type to the user directly in
Job Server, as we do for certain specific kinds of database error.

Author: evansd

ehrql/__main__.py

@@ -124,14 +124,14 @@ def main(args, environ=None):
         # Errors from definition files are already pre-formatted so we just write them
         # directly to stderr and exit
         print(str(exc), file=sys.stderr)
-        sys.exit(1)
+        sys.exit(10)
     except FileValidationError as exc:
         # Handle errors encountered while reading user-supplied data
         print(f"{exc.__class__.__name__}: {exc}", file=sys.stderr)
-        sys.exit(1)
+        sys.exit(11)
     except EHRQLPermissionError as exc:
         print(f"{exc.__class__.__name__}: {exc}", file=sys.stderr)
-        sys.exit(1)
+        sys.exit(12)
     except Exception as exc:
         # For functions which take a `backend_class` give that class the chance to set
         # the appropriate exit status for any errors

tests/functional/test_dump_dataset_sql.py

@@ -78,7 +78,7 @@ def dataset_definition():
     with pytest.raises(SystemExit) as exc_info:
         call_cli("dump-dataset-sql", dataset_definition_path)
 
-    assert exc_info.value.code == 1
+    assert exc_info.value.code > 0
     captured = call_cli.readouterr()
     assert "patients.date_of_birth.year + (patients.sex.is_null())" in captured.err
     assert "main.py" not in captured.err