zerver: API to create channel.

Fixes #16206.

Co-authored-by: Sahil Batra <sahil@zulip.com>
Co-authored-by: Steve Howell <showell@zulip.com>
Signed-off-by: apoorvapendse <apoorvavpendse@gmail.com>

zerver/lib/exceptions.py

@@ -17,6 +17,7 @@ class ErrorCode(Enum):
     BAD_NARROW = auto()
     CANNOT_DEACTIVATE_LAST_USER = auto()
     MISSING_HTTP_EVENT_HEADER = auto()
+    CHANNEL_ALREADY_EXISTS = auto()
     STREAM_DOES_NOT_EXIST = auto()
     UNAUTHORIZED_PRINCIPAL = auto()
     UNSUPPORTED_WEBHOOK_EVENT_TYPE = auto()
@@ -174,6 +175,20 @@ class UnauthorizedError(JsonableError):
         return extra_headers_dict
 
 
+class ChannelExistsError(JsonableError):
+    code = ErrorCode.CHANNEL_ALREADY_EXISTS
+    http_status_code = 409
+    data_fields = ["channel_name"]
+
+    def __init__(self, channel_name: str) -> None:
+        self.channel_name = channel_name
+
+    @staticmethod
+    @override
+    def msg_format() -> str:
+        return _("Channel '{channel_name}' already exists")
+
+
 class StreamDoesNotExistError(JsonableError):
     code = ErrorCode.STREAM_DOES_NOT_EXIST
     data_fields = ["stream"]