Bug: HTTP error codes are not being passed through to the SDK level

[bwcDvorak]

Describe the bug

Recent testing by @alexrisch uncovered that message sending errors are not currently bubbling up 429's (and other?) errors up to the SDK layer.

As an example, this prevents someone using the SDK from easily detecting the 429's that indicate they're being rate limited.

Expected behavior

429's and other error codes would bubble up so the SDK user can make use of the info.

Steps to reproduce the bug

No response

[neekolas]

There's been a long-standing ask for us to send a JSON response from our WAF when it rate limits you, instead of a string. That would let us get better errors in the JS SDK.