fix: add a check for partial response data

[cojenco]

Handle corner cases of incomplete downloads with a 200 response code. These are rare cases when a download is prematurely terminated due to network stability or bandwidth throttling etc, but does not result in connection reset errors. The library will

• (1) compare the # bytes read vs x-goog-stored-content-length header
• (2) raise a retry-able ConnectionError in the aim to trigger a retry

Add more context to checksum mismatch error message that will help with debugging

• bytes read from the download request (tracked within the Download object)
• X-Goog-Stored-Content-Length value from the response header
• X-Goog-Stored-Content-Encoding value from the response header

Fixes internal b/278011003

[danielduhh]

Is this good to merge? @cojenco