Raise RuntimeError when first-chunk callback runs without a task

Replace the assert with a RuntimeError to match the codebase's
idiomatic handling of programmer errors, per PR review feedback.

Signed-off-by: Stephen Crowe <6042774+crowecawcaw@users.noreply.github.com>

Author: crowecawcaw

s3transfer/download.py

@@ -661,10 +661,10 @@ def __init__(
         self._task = None
 
     def __call__(self):
-        # Always check if we have a task and response first
-        assert self._task is not None, (
-            "set_task() must be called before the task is submitted"
-        )
+        if self._task is None:
+            raise RuntimeError(
+                "set_task() must be called before the task is submitted"
+            )
 
         response = self._task.get_response()
         if not response: