Improve hints around `flag create`

[khvn26]

Testers report confusion upon seeing this message:

matvei@macstream ~ % flagsmith flag list
No flags.
matvei@macstream ~ % flagsmith flag create
Error: flags exist per environment, so there is nothing to create
To create the feature itself, run `flagsmith feature create <feature>`.

It seems we can do a better job directing users to feature create, e.g.:

matvei@macstream ~ % flagsmith flag create
Error: flags can only be changed, not created.
To create a feature, run `flagsmith feature create <feature>`.
To change a flag, run `flagsmith flag update <feature>`.