--format should return an error if the format isn't known

[lucapette]

Right now, we do not check if the input format is known. That leads to a strange interaction:

$ fakedata -f=unknown email
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]

We default to the space separated format, therefore the current behavior. It's rather odd (and hard to notice) if the command has a small typo. Example:

$ fakedata -f=tav -l=1 email ipv4
[email protected] 239.201.152.149

The output contains spaces instead of tabs because the input format was tav.