Make HTML headers dynamic and responsive to a user's machine.

[schmidtDETR]

Currently, fread_bls headers are static. They would provide better information if they were rendered dynamically (for example, based on the user's OS.

[schmidtDETR]

Bumping this and adding bug label as the headers as currently structured have started causing issues with the BLS website and blocking access. Suggested improvements:

Make the User-Agent header a users email address (could be satisfied with setting an environment variable?)
When access fails, include the body of the response (not just the response code) to allow for better debugging.