[ansible] Ensure user/group selected for macOS systemwide installatio…

…n actually exist (#479)

tasks/pkg-macos.yml

@@ -6,6 +6,17 @@
     msg: "The Datadog ansible role does not currently support Agent 5 on macOS"
   when: datadog_agent_major_version|int == 5
 
+- name: Check if the macOS user for Agent service exists
+  command: id -u "{{ datadog_macos_user }}"
+  register: mac_user_check
+  changed_when: false
+  ignore_errors: true
+
+- name: Fail if the macOS user for Agent service doesn't exist
+  fail:
+    msg: "The Datadog ansible role wasn't able to find the user : {{ datadog_macos_user }}"
+  when: mac_user_check.rc != 0
+
 - include_tasks: pkg-macos/macos_agent_latest.yml
   when: (not datadog_skip_install) and (datadog_agent_macos_version is not defined)