Make minor correctness and display improvements to pull logs

Summary:
Depends on D18915. Ref T13046.

• Distinguish between HTTP and HTTPS.
• Use more constants and fewer magical strings.
• For HTTP responses, give them better type information and more helpful UI behaviors.

Test Plan: Pulled over SSH and HTTP. Reviewed resulting logs from the web UI. Hit errors like missing/invalid credentials.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13046

Differential Revision: https://secure.phabricator.com/D18917