[v2] Incorrect message if `keyfile` specified in nvchecker.toml is missing

[yan12125]

$ cat nvchecker.toml         
[__config__]
keyfile = "/does/not/exist"

$ nvchecker -c nvchecker.toml
version configuration file not given and default does not exist

I didn't check 1.x and thus I'm not sure if this is a regression or not.

[lilydjwg]

Thanks!