webhooks: Log unexpected webhook events separately.

This change serves to declutter webhook-errors.log, which is
filled with too many UnexpectedWebhookEventType exceptions.

Keeping UnexpectedWebhookEventType in zerver/lib/webhooks/common.py
led to a cyclic import when we tried to import the exception in
zerver/decorators.py, so this commit also moves this exception to
another appropriate module. Note that our webhooks still import
this exception via zerver/lib/webhooks/common.py.

zerver/lib/exceptions.py

@@ -1,10 +1,11 @@
 from enum import Enum
-from typing import Any, Dict, List, Type
+from typing import Any, Dict, List, Type, Optional
 from mypy_extensions import NoReturn
 
 from django.core.exceptions import PermissionDenied
 from django.utils.translation import ugettext as _
 
+
 class AbstractEnum(Enum):
     '''An enumeration whose members are used strictly for their names.'''
 
@@ -200,3 +201,15 @@ class InvalidAPIKeyError(JsonableError):
     @staticmethod
     def msg_format() -> str:
         return _("Invalid API key")
+
+class UnexpectedWebhookEventType(JsonableError):
+    code = ErrorCode.UNEXPECTED_WEBHOOK_EVENT_TYPE
+    data_fields = ['webhook_name', 'event_type']
+
+    def __init__(self, webhook_name: str, event_type: Optional[str]) -> None:
+        self.webhook_name = webhook_name
+        self.event_type = event_type
+
+    @staticmethod
+    def msg_format() -> str:
+        return _("The '{event_type}' event isn't currently supported by the {webhook_name} webhook")