maas boot-resources create typo error message is unhelpful

[Ian Johnson]

While trying to create a custom image in maas, I had <email address hidden> when it should have been content@=foo.tgz, but the only error message maas gives me when doing this is:
```
usage: maas [-h] COMMAND ...

optional arguments:
  -h, --help show this help message and exit

drill down:
  COMMAND
    login Log in to a remote API, and remember its description and credentials.
    logout Log out of a remote API, purging any stored credentials.
    list List remote APIs that have been logged-in to.
    refresh Refresh the API descriptions of all profiles.
    init Initialise MAAS in the specified run mode.
    config View or change controller configuration.
    status Status of controller services.
    migrate Perform migrations on connected database.
    apikey Used to manage a user's API keys. Shows existing keys unless --generate or --delete is passed.
    configauth Configure external authentication.
    createadmin Create a MAAS administrator account.
    changepassword
                  Change a MAAS user's password.
    admin Interact with http://localhost:5240/MAAS/api/2.0/

http://maas.io/

'str' object is not callable
```

Which made me very confused and didn't make clear that the issue is with the formatting of the content parameter and not with the commands I was providing. Something not being callable made me think I had typed the commands wrong or something like that

[Jerzy Husakowski (jhusakowski)]

To be picked up together with cli improvements.