fix: don't print success before error

[mkenigs]

Running flox activate twice was printing:

✅ You are now using the environment 'name'.
To stop using this environment, type 'exit'

ERROR: Environment 'name' is already active.

Remove the success message from Rust, and only print it in Bash codepaths when the environment is not already active (and an interactive activation is being run).

Also add a ❌ to the error message.