Raise better error types

Author: cpsievert

shiny/ui/_chat_tokenizer.py

@@ -51,14 +51,14 @@ def get_default_tokenizer() -> TokenizersTokenizer:
 
         return Tokenizer.from_pretrained("bert-base-cased")  # type: ignore
     except ImportError:
-        raise ValueError(
+        raise ImportError(
             "Failed to download a default tokenizer. "
             "A tokenizer is required to impose `token_limits` on `chat.messages()`. "
             "To get a generic default tokenizer, install the `tokenizers` "
             "package (`pip install tokenizers`). "
         )
     except Exception as e:
-        raise ValueError(
+        raise RuntimeError(
             "Failed to download a default tokenizer. "
             "A tokenizer is required to impose `token_limits` on `chat.messages()`. "
             "Try downloading a different tokenizer using "