Add custom exceptions for each error state.

The bot can get into trouble in three distinct ways:
- It has no Bot instance
- It has no namespace
- It has no parent instance.

These happen only if you're using it wrong. To make the test more
precise, and to add a little bit more readability (RuntimeError could be
anything!), we'll introduce some custom exceptions for these three
states.

This addresses a review comment by @aeros.

1 files changed, 4 insertions, 3 deletions

diff --git a/tests/bot/utils/test_redis_cache.py b/tests/bot/utils/test_redis_cache.py
index 4f95dff03..8c1a40640 100644
--- a/tests/bot/utils/test_redis_cache.py
+++ b/tests/bot/utils/test_redis_cache.py
@@ -4,6 +4,7 @@ import unittest
 import fakeredis.aioredis
 
 from bot.utils import RedisCache
+from bot.utils.redis_cache import NoBotInstanceError, NoNamespaceError, NoParentInstanceError
 from tests import helpers
 
 
@@ -260,13 +261,13 @@ class RedisCacheTests(unittest.IsolatedAsyncioTestCase):
         cog = MyCog()
 
         # Raises "No Bot instance"
-        with self.assertRaises(RuntimeError):
+        with self.assertRaises(NoBotInstanceError):
             await cog.cache.get("john")
 
         # Raises "RedisCache has no namespace"
-        with self.assertRaises(RuntimeError):
+        with self.assertRaises(NoNamespaceError):
             await cog.other_cache.get("was")
 
         # Raises "You must access the RedisCache instance through the cog instance"
-        with self.assertRaises(RuntimeError):
+        with self.assertRaises(NoParentInstanceError):
             await MyCog.cache.get("afraid")