webhook-errors.log file is cluttered with Stream.DoesNotExist
errors, which hides the errors that we actually need to see. So,
since check_message already sends the bot_owner a PM if the webhook
bot tries to send a message to a non-existent stream, we can ignore
such exceptions.
This commit adds a generic function called check_send_webhook_message
that does the following:
* If a stream is specified in the webhook URL, it sends a stream
message, otherwise sends a PM to the owner of the bot.
* In the case of a stream message, if a custom topic is specified
in the webhook URL, it uses that topic as the subject of the
stream message.
Also, note that we need not test this anywhere except for the
helloworld webhook. Since helloworld is our default example for
webhooks, it is here to stay and it made sense that tests for a
generic function such as check_send_webhook_message be tested
with an actual generic webhook!
Fixes#8607.
Eeshan Garg