Report read errors correctly

[mattjala]

GET_Value and POST_Value sent the success response headers before fetching any data, then caught every error from the fetch so they could still finish the response. A read that failed on the data node therefore came back as:

• a JSON read: 200 with an empty body
• a binary read with a Content-Length: 200, then fewer bytes than promised, which clients report as a broken connection
• a streamed read without a Content-Length (queries, variable-length types): 200 and a normally terminated body containing whatever pages were written before the failure. In testing, a query over a dataset with a bad chunk returned 2000 of its 4000 bytes as a success.

Both handlers now fetch before sending headers, and any errors there propagate. A failure on a later page can't change the status, so the handler raises instead of ending the stream, and aiohttp drops the connection so that the client sees an incomplete transfer instead of a complete-looking short one. JSON serialization errors also happen before the headers now, where they used to be swallowed the same way.