YAML errors cause security policy to fail executing

https://sentry.gitlab.net/gitlab/gitlabcom/issues/3008584/?referrer=gitlab_plugin

Psych::SyntaxError: (<unknown>): did not find expected alphabetic or numeric character while scanning an alias at line 28 column 14
  lib/gitlab/config/loader/yaml.rb:13:in `initialize'
    @config = YAML.safe_load(config,
  lib/gitlab/utils/strong_memoize.rb:30:in `strong_memoize'
    instance_variable_set(ivar(name), yield)
  app/models/concerns/each_batch.rb:98:in `block (2 levels) in each_batch'
    unscoped { yield relation, index }
  app/models/concerns/each_batch.rb:98:in `block in each_batch'
    unscoped { yield relation, index }
  app/models/concerns/each_batch.rb:68:in `step'
    1.step do |index|
...
(113 additional frame(s) were not displayed)

Gitlab::Config::Loader::FormatError: (<unknown>): did not find expected alphabetic or numeric character while scanning an alias at line 28 column 14
  lib/gitlab/config/loader/yaml.rb:19:in `rescue in initialize'
    raise Loader::FormatError, e.message
  lib/gitlab/config/loader/yaml.rb:12:in `initialize'
    def initialize(config, additional_permitted_classes: [])
  lib/gitlab/utils/strong_memoize.rb:30:in `strong_memoize'
    instance_variable_set(ivar(name), yield)
  app/models/concerns/each_batch.rb:98:in `block (2 levels) in each_batch'
    unscoped { yield relation, index }
  app/models/concerns/each_batch.rb:98:in `block in each_batch'
    unscoped { yield relation, index }
...
(110 additional frame(s) were not displayed)

(<unknown>): did not find expected alphabetic or numeric character while scanning an alias at line 28 column 14