Ignore import errors in API URLs check

arkindex/project/checks.py

@@ -38,7 +38,16 @@ def api_urls_check(*args, **kwargs):
     except those that end with a <path:…> parameter,
     since the ending slash would become part of the parameter.
     '''
-    from arkindex.project.api_v1 import api
+    try:
+        from arkindex.project.api_v1 import api
+    except ImportError as e:
+        return [
+            Error(
+                f'Cannot import API URLs: {e}',
+                id='arkindex.E001',
+            )
+        ]
+
     return [
         Error(
             'This API URL misses a mandatory ending /',