[elm] add function to allow more fine grained error handling

Created by: 7omb

This is a backwards compatible change. It adds the additional function sendWithCustomExpect to the provided helper functions in Api.elm to make requests.

It can be used to implement a more fine grained error handling as described here. Before it was e.g. not possible to access the response body if the server responded with an error like 404.

@eriktim

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