better error message for fuzzit auth

[kjk]

Currently:

$ fuzzit auth
Error: accepts 1 arg(s), received 0
...

Error message should be more descriptive, like:

Please provide api key. To create api key go to https://app.fuzzit.dev/settings and use "Generate New Key" button.

Similarly:

$ fuzzit help auth
Authenticate with fuzzit servers

Usage:
  Fuzzit auth [api_key] [flags]

Flags:
  -h, --help   help for auth

It could be:

$ fuzzit help auth
Authenticate with fuzzit servers with api key. To create api key go to https://app.fuzzit.dev/settings and use "Generate New Key" button.

Usage:
  Fuzzit auth [api_key] [flags]

Flags:
  -h, --help   help for auth

[yevgenypats]

done