Mention specifically which account config has an error

[nkakouros]

Now, when an assertion on the contents of offlineimap_accounts fails, it does not mention the account that the assertion failed, making it a bit hard for the user to understand where exactly the error is.

The name of the failed account should also be returned. For this to happen and to avoid writing complex jinja2 templates, a custom filter should be used to read the offlineimap_accounts variable and return:

[
  [account1.name, account1.key1],
  [account1.name, account1.key2],
  ...,
  [account2.name, account2.key1],
  [account2.name, account2.key2],
  ...,
]

I haven't managed to achieve this with existing Ansible filters.