Delete constraints and requirements files on build fail

If the installation of Python packages fails for any reason, the
constraint and requirements file is already written and therefore
unless you delete the virtualenv, it would not rebuild things
again.

This patch will handle failure and delete those files, then provide
a warning for the user upon failing to inspect the needed file.

Closes-Bug: #1824189

Change-Id: I67db44ce10250908f23c26c90d271a780a875eb6

tasks/python_venv_install.yml

@@ -110,23 +110,35 @@
     - venv changed
 
 - name: Install python packages into the venv
-  pip:
-    name: "{{ (venv_default_pip_packages | union(venv_pip_packages)) | sort }}"
-    state: "{{ venv_pip_package_state }}"
-    virtualenv: "{{ venv_install_destination_path }}"
-    extra_args: >-
-      --constraint {{ venv_install_destination_path }}/constraints.txt
-      --pre
-      --log /var/log/python_venv_build.log
-      {{ venv_default_pip_install_args }}
-      {{ venv_pip_install_args }}
-  when: (_requirement_file is changed) or (_constraint_file is changed)
-  register: _install_venv_pip_packages
-  until: _install_venv_pip_packages is success
-  retries: 5
-  delay: 2
-  notify:
-    - venv changed
+  block:
+    - pip:
+        name: "{{ (venv_default_pip_packages | union(venv_pip_packages)) | sort }}"
+        state: "{{ venv_pip_package_state }}"
+        virtualenv: "{{ venv_install_destination_path }}"
+        extra_args: >-
+          --constraint {{ venv_install_destination_path }}/constraints.txt
+          --pre
+          --log /var/log/python_venv_build.log
+          {{ venv_default_pip_install_args }}
+          {{ venv_pip_install_args }}
+      when: (_requirement_file is changed) or (_constraint_file is changed)
+      register: _install_venv_pip_packages
+      until: _install_venv_pip_packages is success
+      retries: 5
+      delay: 2
+      notify:
+        - venv changed
+  rescue:
+    - file:
+        path: "{{ item }}"
+        state: absent
+      with_items:
+        - "{{ venv_install_destination_path }}/constraints.txt"
+        - "{{ venv_install_destination_path }}/requirements.txt"
+    - fail:
+        msg: >
+          The python packages have failed to install, please check the log file
+          located at /var/log/python_venv_build.log for more information.
 
 - name: Add symlinks from distribution python packages
   when: