Detect/report possible rate limit reached

It turns out that GitHub APIs have a rate limit: this breaks Bower after a large number of frequent installs/updates. At least detect and report that properly.
Manually determine final redirected URL for GitHub's API URLs

GitHub's regular URLs do redirect when a repo has been moved, but its API URLs do not (e.g. https://api.github.com/repos/Polymer/polymer/tags). Redirect manually.
Bower: improve error reseliense & messaging
Address review comments
Merge pull request #3690 from ussuri/bower-improve-stability

Bower: handle moved repos, improve stability & messaging