[bugfix] Improve log level and info for custom ops build (#937)

### What this PR does / why we need it?
Fix the bug of #703, where vllm wrong raised the ERROR : Failed to
import vllm_ascend_C:No module named 'vllm_ascend.vllm_ascend_C'. The
format for reporting import vllm_ascend_C failure is unified by warning
("Failed to import vllm_ascend_C:%s", e).

### Does this PR introduce _any_ user-facing change?
No

---------

Signed-off-by: yangpuPKU <604425840@qq.com>

vllm_ascend/device_allocator/camem.py

@@ -60,7 +60,8 @@ try:
     lib_name = find_loaded_library("vllm_ascend_C")
     camem_available = True
 except ImportError as e:
-    logger.error("Failed to import vllm_ascend_C:%s", e)
+    logger.warning(
+        "Failed to import vllm_ascend_C:%s. Sleep mode will be disabled. ", e)
     init_module = None
     python_create_and_map = None
     python_unmap_and_release = None