Handle `ElasticsearchSearchBackend` index exceptions the way it should

For instance, it was nearly impossible to use commands like
`update_index` or `rebuild_index` when `silently_fail` was set to False,
because of lacking of exception handling.
Improve IndexAlreadyExistsError exception support

since some third party services do not format it the way pyelasticsearch
wants
Add tests for a non-silent elasticsearch backend
Improve exception logging when silently_fail is True
Add Pierre Dulac to the AUTHORS
Improve `ElasticsearchSearchBackendNonSilentTestCase` tests
Wait for cluster to be ready before setup ends
Merge branch 'fix_875'

Conflicts:
	AUTHORS
	haystack/backends/elasticsearch_backend.py