do not hide errors when importing tokenizer

Explicitly check for the tokenizer source file to check that
the name is correct. We can't use the import error for that
because it hides other import errors like a missing
library.

Fixes #2327.

nominatim/tokenizer/factory.py

@@ -15,6 +15,7 @@ normalizer module is installed, when the tokenizer is created.
 """
 import logging
 import importlib
+from pathlib import Path
 
 from ..errors import UsageError
 from ..db import properties
@@ -25,12 +26,13 @@ LOG = logging.getLogger()
 def _import_tokenizer(name):
     """ Load the tokenizer.py module from project directory.
     """
-    try:
-        return importlib.import_module('nominatim.tokenizer.' + name + '_tokenizer')
-    except ModuleNotFoundError as exp:
+    src_file = Path(__file__).parent / (name + '_tokenizer.py')
+    if not src_file.is_file():
         LOG.fatal("No tokenizer named '%s' available. "
                   "Check the setting of NOMINATIM_TOKENIZER.", name)
-        raise UsageError('Tokenizer not found') from exp
+        raise UsageError('Tokenizer not found')
+
+    return importlib.import_module('nominatim.tokenizer.' + name + '_tokenizer')
 
 
 def create_tokenizer(config, init_db=True, module_name=None):

test/python/conftest.py

@@ -17,6 +17,7 @@ from nominatim.config import Configuration
 from nominatim.db import connection
 from nominatim.db.sql_preprocessor import SQLPreprocessor
 from nominatim.db import properties
+import nominatim.tokenizer.factory
 
 import dummy_tokenizer
 import mocks
@@ -273,7 +274,7 @@ def tokenizer_mock(monkeypatch, property_table, temp_db_conn, tmp_path):
     def _import_dummy(module, *args, **kwargs):
         return dummy_tokenizer
 
-    monkeypatch.setattr(importlib, "import_module", _import_dummy)
+    monkeypatch.setattr(nominatim.tokenizer.factory, "_import_tokenizer", _import_dummy)
     properties.set_property(temp_db_conn, 'tokenizer', 'dummy')
 
     def _create_tokenizer():