Improve exception type.

ImportError isn't really appropriate when there's no import involved.

src/transformers/commands/serving.py

@@ -107,7 +107,7 @@ class ServeCommand(BaseTransformersCLICommand):
         self._host = host
         self._port = port
         if not _serve_dependancies_installed:
-            raise ImportError(
+            raise RuntimeError(
                 "Using serve command requires FastAPI and unicorn. "
                 "Please install transformers with [serving]: pip install transformers[serving]."
                 "Or install FastAPI and unicorn separatly."