KeyError: 'retry_after' when hitting excessive ratelimits for webhooks.

[mathsman5133]

Summary

I've noticed sometimes when sending logs to discord via webhooks that hitting the ratelimits excessively (yes, bad design on my part and it's fixed, but nonetheless a valid bug report I feel) raises a KeyError when looking for the time to retry after. I did a bit of digging and pulled up this as the response payload:

{
code: 0, 
message: 'You are being blocked from accessing our API temporarily due to exceeding our rate limits frequently. Please read our docs at https://discord.com/developers/docs/topics/rate-limits to prevent this moving forward.'
}

Would it be helpful to forward this error to the user?

Reproduction Steps

import discord
import asyncio
import aiohttp

webhook = discord.Webhook.from_url("webhook url", adapter=discord.AsyncWebhookAdapter(session=aiohttp.ClientSession()))

async def suppress_exc():
    try:
        await webhook.send("test")
    except discord.HTTPException:
        await suppress_exc()  # we just want to make more requests that hit the ratelimit


async def test():
    tasks = []
    for _ in range(60):
        tasks.append(suppress_exc())

    await asyncio.gather(*tasks)

asyncio.get_event_loop().run_until_complete(test())

Expected Results

Tell me I shouldn't try to abuse ratelimits

Actual Results

Traceback (most recent call last):
  File "C:/Users/User/py/donationbot/webhook_bug.py", line 21, in <module>
    asyncio.get_event_loop().run_until_complete(test())
  File "C:\Users\User\AppData\Local\Programs\Python\Python37-32\lib\asyncio\base_events.py", line 584, in run_until_complete
    return future.result()
  File "C:/Users/User/py/donationbot/webhook_bug.py", line 19, in test
    await asyncio.gather(*tasks)
  File "C:/Users/User/py/donationbot/webhook_bug.py", line 9, in suppress_exc
    await webhook.send("test")
  File "C:\Users\User\.virtualenvs\donationbot\lib\site-packages\discord\webhook.py", line 236, in handle_execution_response
    data = await response
  File "C:\Users\User\.virtualenvs\donationbot\lib\site-packages\discord\webhook.py", line 218, in request
    retry_after = response['retry_after'] / 1000.0
KeyError: 'retry_after'

Checklist

• I have searched the open issues for duplicates.
• I have shown the entire traceback, if possible.
• I have removed my token from display, if visible.

System Information

• Python v3.7.3-final
• discord.py v1.4.1-final
• aiohttp v3.6.2
• system info: Windows 10 10.0.18362

[wildwastaken]

@Rapptz how did you fix this?

[Gobot1234]

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