Executor crashes when failing to authorize against a rule mentioning multiple permissions

[christophermaier]

When failing to authenticate against a rule that mentions multiple permissions, Cog will crash when rendering the error message. All proper authorization checks have taken place, but no output is returned to the user.

The offending code is at

cog/lib/cog/error_response.ex

Lines 15 to 16 in a793edb

	def render({:denied, {%Ast.Rule{}=rule, current_invocation}}),
	do: "Sorry, you aren't allowed to execute '#{current_invocation}' :(\n You will need the '#{rule.permission_selector.perms.value}' permission to run this command."

An example of a rule that will trigger this condition is:

when command is twitter:tweet with option[as] == "support" must have twitter:tweet and site:support

If your user does not have both permissions, executing

twitter:tweet --as=support "blah blah blah"

will crash the executor process.

The error rendering code was written assuming a rule would only mention a single permission. This specific failure is caused because the AST node in question is now a Piper.Permissions.Ast.ConditionalExpr, and it has no :perms field.