Update the docs and explicitly specify missing result error

Author: booqoffsky

README.md

@@ -18,6 +18,7 @@ pip install taskiq-redis
 Let's see the example with the redis broker and redis async result:
 
 ```python
+# broker.py
 import asyncio
 
 from taskiq_redis import ListQueueBroker, RedisAsyncResultBackend
@@ -42,12 +43,18 @@ async def best_task_ever() -> None:
 
 async def main():
     task = await best_task_ever.kiq()
-    print(await task.get_result())
+    print(await task.wait_result())
 
 
-asyncio.run(main())
+if __name__ == "__main__":
+    asyncio.run(main())
 ```
 
+Launch the workers:
+`taskiq worker broker:broker`
+Then run the main code:
+`python3 broker.py`
+
 ## PubSubBroker and ListQueueBroker configuration
 
 We have two brokers with similar interfaces, but with different logic.

taskiq_redis/exceptions.py

@@ -8,3 +8,7 @@ class DuplicateExpireTimeSelectedError(TaskIQRedisError):
 
 class ExpireTimeMustBeMoreThanZeroError(TaskIQRedisError):
     """Error if two lifetimes are less or equal zero."""
+
+
+class ResultIsMissingError(TaskIQRedisError):
+    """Error if there is no result when trying to get it."""

taskiq_redis/redis_backend.py

@@ -8,6 +8,7 @@
 from taskiq_redis.exceptions import (
     DuplicateExpireTimeSelectedError,
     ExpireTimeMustBeMoreThanZeroError,
+    ResultIsMissingError,
 )
 
 _ReturnType = TypeVar("_ReturnType")
@@ -121,6 +122,9 @@ async def get_result(  # noqa: WPS210
                     name=task_id,
                 )
 
+        if result_value is None:
+            raise ResultIsMissingError()
+
         taskiq_result: TaskiqResult[_ReturnType] = pickle.loads(result_value)
 
         if not with_logs: