Remove index for which the base_url failed

Problem description
================

If an index is not available, pip will continue to try the index when installing each file.

When installing a long requirements file, the whole timeouts take a long time.

It would be nice if pip could remove the index and stop trying it.

This can be useful for development and when running continuous integration together with --find-links=file:/// . In case the index is down, pip will just use the cached files.

This can simplifies the build system, since there is no need to look for index failures and then to retry without the index.

I know that index lookup can be explicitly turned off, so I am not sure if this behavior is wanted.


Changes
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When get_page will fail for index's base URL, the index url will be removed from the list of indexes.


How to test
=========

There are no tests for now, since I don't know if such behavior is wanted in pip.

I will add the required tests.

Thanks!