Expose error when checking if alias exists

[cwurm]

We are using a HEAD /_alias/{alias} request to check if a particular alias exists during setup. If this request fails (e.g. because of missing privileges) we will not get the full error message from Elasticsearch telling us which privilege is missing. Changing to GET will get us that while maintaining functionality.

Before (when missing privileges):

2019-07-12T10:29:43.672+0100	ERROR	instance/beat.go:877	Exiting: failed to check for alias 'auditbeat-7.2.0': (status=403) : 403 Forbidden:
Exiting: failed to check for alias 'auditbeat-7.2.0': (status=403) : 403 Forbidden:

After:

Exiting: failed to check for alias 'auditbeat-8.0.0': (status=403) {"error":{"root_cause":[{"type":"security_exception","reason":"action [indices:admin/aliases/get] is unauthorized for user [auditbeat]"}],"type":"security_exception","reason":"action [indices:admin/aliases/get] is unauthorized for user [auditbeat]"},"status":403}: 403 Forbidden: {"error":{"root_cause":[{"type":"security_exception","reason":"action [indices:admin/aliases/get] is unauthorized for user [auditbeat]"}],"type":"security_exception","reason":"action [indices:admin/aliases/get] is unauthorized for user [auditbeat]"},"status":403}

Also seen in #10241 (comment).

[andrewkroh]

I think this should have a changelog entry.