[exporter/otlphttp] Partial success handlers should return error when invalid content type is returned

[tvaintrob]

Describe the bug

Currently when an invalid content-type is returned to the partial success handlers a nil is returned, an error should be returned instead

What did you expect to see?

An error returned

What did you see instead?

A nil is returned

What version did you use?

Latest main

[atoulme]

Thanks for the report Tal! Would you like to work on this? A failing unit test would help speed up delivery of a fix.

[TylerHelmuth]

@atoulme this issue is a result of #9276 (comment). We're aware but will do the functionality change it is own PR.