Wrap error from account alias permissions error

Per envato#362, the returned error message doesn't explain why the IAM
permission was required. While the wrapped error doesn't mention the
specific permission, the original MissingIamPermissionsError can still
be seen in --trace output, as it is registered as the Error#cause

lib/stack_master/identity.rb

@@ -1,12 +1,15 @@
 module StackMaster
   class Identity
+    AllowedAccountAliasesError = Class.new(StandardError)
     MissingIamPermissionsError = Class.new(StandardError)
 
     def running_in_account?(accounts)
       accounts.nil? ||
         accounts.empty? ||
         contains_account_id?(accounts) ||
         contains_account_alias?(accounts)
+    rescue MissingIamPermissionsError
+      raise AllowedAccountAliasesError, "Unable to validate whether the current AWS account is allowed"
     end
 
     def account