Error message for invalid 'content-type' in subgraph response doesn't provide enough information

[IvanGoncharov]

Describe the bug
At the moment error message for invalid 'content-type' in subgraph response looks like this:

HTTP fetch failed from 'test': subgraph didn't return JSON (expected content-type: application/json or content-type: application/graphql-response+json; found content-type: text/html)

It doesn't contain invalid values, and it doesn't have any explanation of what is wrong with a content type.