[suiop][env] fix messaging, remove error

[after-ephemera]

Description

• remove old error checking for an env file
• add a new message to indicate that a subshell is being created

Test plan

suiop-cli [jkj/suiop-env-msg-fix] suiop env                    
> Select an environment: mysten/default/vultr-api
Starting a subshell with the environment set... exit the shell (usually ctrl-d) to exit the env
error This project's package.json defines "packageManager": "yarn@[email protected]". However the current global version of Yarn is 1.22.22.                                                                                                                                                                                

Presence of the "packageManager" field indicates that the project is meant to be used with Corepack, a tool included by default with all official Node.js distributions starting from 16.9 and 14.19.
Corepack must currently be enabled by running corepack enable in your terminal. For more information, check out https://yarnpkg.com/corepack.
done initializing 🎉
suiop-cli [jkj/suiop-env-msg-fix] 

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