Add better error messages: If backend dependencies are missing, captu…

…re original ImportError and delay raising until class is initialized

aiohttp_client_cache/backends/__init__.py

@@ -1,28 +1,46 @@
+from logging import getLogger
+
 from aiohttp_client_cache.backends.base import (  # noqa: F401
     BaseCache,
     CacheBackend,
     DictCache,
     ResponseOrKey,
 )
 
+logger = getLogger(__name__)
+
+
+def get_placeholder_backend(original_exception):
+    """This creates a placeholder type for a backend class that does not have dependencies
+    installed. This allows delaying the ImportError until init is called, rather then when imported.
+    """
+
+    class PlaceholderBackend:
+        def __init__(*args, **kwargs):
+            logger.error('Dependencies are not installed for this backend')
+            raise original_exception
+
+    return PlaceholderBackend
+
+
 # Import all backends for which dependencies are installed
 try:
     from aiohttp_client_cache.backends.dynamodb import DynamoDBBackend
-except ImportError:
-    DynamoDBBackend = None  # type: ignore
+except ImportError as e:
+    DynamoDBBackend = get_placeholder_backend(e)  # type: ignore
 try:
     from aiohttp_client_cache.backends.gridfs import GridFSBackend
-except ImportError:
-    GridFSBackend = None  # type: ignore
+except ImportError as e:
+    GridFSBackend = get_placeholder_backend(e)  # type: ignore
 try:
     from aiohttp_client_cache.backends.mongo import MongoDBBackend
-except ImportError:
-    MongoDBBackend = None  # type: ignore
+except ImportError as e:
+    MongoDBBackend = get_placeholder_backend(e)  # type: ignore
 try:
     from aiohttp_client_cache.backends.redis import RedisBackend
-except ImportError:
-    RedisBackend = None  # type: ignore
+except ImportError as e:
+    RedisBackend = get_placeholder_backend(e)  # type: ignore
 try:
     from aiohttp_client_cache.backends.sqlite import SQLiteBackend
-except ImportError:
-    SQLiteBackend = None  # type: ignore
+except ImportError as e:
+    SQLiteBackend = get_placeholder_backend(e)  # type: ignore