Return 400 on Twilio errors

[sethvargo]

These are nearly always client errors

Fixes #1310

Release Note

Return 400 (instead of 500) on Twilio errors

[sethvargo]

/assign @whaught

I thought about parsing the code, but there are over 10k possible codes, and only 14 of them are internal server errors: https://www.twilio.com/docs/api/errors

[whaught]

Well this works.

I wonder if we want to go the other way and select a few eg. 503 unknown to keep as Server error

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[whaught]

/assign @whaught

I thought about parsing the code, but there are over 10k possible codes, and only 14 of them are internal server errors: https://www.twilio.com/docs/api/errors

Just picking one at random: error code 90002: "too many errors" error. This list is a little ridiculous