[Misc] Improve modelscope import error (#17983)

Signed-off-by: Jee Jee Li <pandaleefree@gmail.com>

vllm/transformers_utils/__init__.py

@@ -3,6 +3,7 @@
 from vllm.envs import VLLM_USE_MODELSCOPE
 
 if VLLM_USE_MODELSCOPE:
+    try:
         # Patch here, before each import happens
         import modelscope
         from packaging import version
@@ -12,8 +13,11 @@ if VLLM_USE_MODELSCOPE:
             raise ImportError(
                 'Using vLLM with ModelScope needs modelscope>=1.18.1, please '
                 'install by `pip install modelscope -U`')
-
         from modelscope.utils.hf_util import patch_hub
 
         # Patch hub to download models from modelscope to speed up.
         patch_hub()
+    except ImportError as err:
+        raise ImportError(
+            "Please install modelscope>=1.18.1 via "
+            "`pip install modelscope>=1.18.1` to use ModelScope.") from err