Remove 'ignore_errors: true' in favor of 'failed_when: false'

This change removes the use of 'ignore_errors: true' because it causes deployers
to see red output and a stacktrace, which traditionally means something is broken,
even when the failure is known to have a fall back option or be intentional. This
conversion will provide a generally cleaner interface.

It should be noted that the 'failed' filter will still function normally. Tasks
with the 'failed_when: false' option will still be marked as 'failed' in any
registered variable. This change simply makes the output look cleaner.

Change-Id: Ia6f55ae6fd6270d03e6bf0541d577cd862b3a16b
Closes-Bug: #1633438
Signed-off-by: Kevin Carter <kevin.carter@rackspace.com>
(cherry picked from commit 3e65a5a166)

tasks/sahara_install.yml

@@ -70,7 +70,7 @@
     url: "{{ sahara_venv_download_url }}"
     dest: "/var/cache/{{ sahara_venv_download_url | basename }}"
     checksum: "sha1:{{ remote_venv_checksum.content }}"
-  ignore_errors: true
+  failed_when: false
   register: get_venv
   when:
     - not sahara_developer_mode | bool