fail to enable encryption on limited cache_level

Encryption shouldn't be supported on limited cache_level. Even if it may
work a bit, it causes a lot of data to be cached.

matrix_client/client.py

@@ -132,6 +132,9 @@ def __init__(self, base_url, token=None, user_id=None,
         if restore_device_id and not encryption:
             raise ValueError("restore_device_id only makes sense when encryption is "
                              "enabled.")
+        if encryption and cache_level != CACHE.ALL:
+            raise ValueError("Encryption is unvailable on cache_level other than "
+                             "CACHE.ALL.")
 
         self.api = MatrixHttpApi(base_url, token)
         self.api.validate_certificate(valid_cert_check)