500 error occurs when an improperly formatted policies.yml file is uploaded

[JonoYang]

When a project is created with a policies.yml file with formatting issues, a 500 error occurs when trying to visit the project page. This is the exception message in the server logs:

web-1     |   File "/opt/scancodeio/.venv/lib/python3.12/site-packages/django/template/defaulttags.py", line 320, in render
web-1     |     match = condition.eval(context)
web-1     |             ^^^^^^^^^^^^^^^^^^^^^^^
web-1     |   File "/opt/scancodeio/.venv/lib/python3.12/site-packages/django/template/defaulttags.py", line 886, in eval
web-1     |     return self.value.resolve(context, ignore_failures=True)
web-1     |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
web-1     |   File "/opt/scancodeio/.venv/lib/python3.12/site-packages/django/template/base.py", line 718, in resolve
web-1     |     obj = self.var.resolve(context)
web-1     |           ^^^^^^^^^^^^^^^^^^^^^^^^^
web-1     |   File "/opt/scancodeio/.venv/lib/python3.12/site-packages/django/template/base.py", line 850, in resolve
web-1     |     value = self._resolve_lookup(context)
web-1     |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
web-1     |   File "/opt/scancodeio/.venv/lib/python3.12/site-packages/django/template/base.py", line 893, in _resolve_lookup
web-1     |     current = getattr(current, bit)
web-1     |               ^^^^^^^^^^^^^^^^^^^^^
web-1     |   File "/opt/scancodeio/scanpipe/models.py", line 1457, in policies_enabled
web-1     |     return bool(self.policy_index)
web-1     |                 ^^^^^^^^^^^^^^^^^
web-1     |   File "/opt/scancodeio/.venv/lib/python3.12/site-packages/django/utils/functional.py", line 47, in __get__
web-1     |     res = instance.__dict__[self.name] = self.func(instance)
web-1     |                                          ^^^^^^^^^^^^^^^^^^^
web-1     |   File "/opt/scancodeio/scanpipe/models.py", line 1452, in policy_index
web-1     |     return self.get_policy_index()
web-1     |            ^^^^^^^^^^^^^^^^^^^^^^^
web-1     |   File "/opt/scancodeio/scanpipe/models.py", line 1443, in get_policy_index
web-1     |     policies_dict = policies.load_policies_file(policies_file)
web-1     |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
web-1     |   File "/opt/scancodeio/scanpipe/policies.py", line 41, in load_policies_file
web-1     |     policies_dict = load_policies_yaml(policies_yaml=policies_file.read_text())
web-1     |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
web-1     |   File "/opt/scancodeio/scanpipe/policies.py", line 33, in load_policies_yaml
web-1     |     raise ValidationError(f"Policies format error: {e}")
web-1     | django.core.exceptions.ValidationError: ['Policies format error: mapping values are not allowed in this context\n  in "<unicode string>", line 53, column 43']

In the case where policies.yml is invalid, we should still be able to see the project page and a message should appear that there is an issue with policies.yml

[tdruez]

Fixed in #1561
The page will now load properly with an error message about the improperly formatted policies.yml file.