[slashtags] fixes a bug related to the "st" dev env value (#158)

* oops typo

* [SlashTags] remove dev env on unload

* [SlashTags] wraps `Red.remove_dev_env_value` in a try except.

* [SlashTags] fix styling

* [SlashTags] try except adding dev env value

slashtags/core.py

@@ -102,7 +102,11 @@ def __init__(self, bot: Red) -> None:
 
         self.load_task = self.create_task(self.initialize_task())
         self.session = aiohttp.ClientSession()
-        bot.add_dev_env_value("st", lambda ctx: self)
+
+        try:
+            bot.add_dev_env_value("st", lambda ctx: self)
+        except Exception:
+            log.exception("Failed to add `slashtags` in the dev environment", exc_info=True)
 
         super().__init__()
 
@@ -128,6 +132,7 @@ def cog_unload(self):
 
     def __unload(self):
         self.bot.remove_dev_env_value("st")
+
         if self.testing_enabled:
             self.remove_test_cog()
         self.load_task.cancel()