When a server action responds with something other than an RSC response,
we currently silently ignore the error and it doesn't get propagated to
any rejection handlers.
This adjusts the handling so that if the server action response is a
non-successful status code, we reject the server action promise. If the
error is `text/plain`, we'll automatically propagate the text content as
the error text. Otherwise, the promise is rejected with a fallback
message.
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