fix(purge): replace CroniterError with CroniterBadCronError

For more accurate error handling.

Author: tigattack

purge/purge.py

@@ -5,7 +5,7 @@
 
 import discord
 from croniter import croniter
-from croniter.croniter import CroniterError
+from croniter.croniter import CroniterBadCronError
 from redbot.core import Config, checks, commands
 
 
@@ -38,7 +38,7 @@ async def set_crontab(self, guild, crontab):
             croniter(crontab)
             await self.config.guild(guild).schedule.set(crontab)
             return crontab
-        except CroniterError:
+        except CroniterBadCronError:
             return False
 
     async def check_purgeable_users(self):