fix: handle redundant deletion of Namespace

[kumar-mallikarjuna]

What this PR does / why we need it:
Adds an IsNotFound check before attempting to delete the Case's Namespace.

Fixes #565

[porridge]

Sorry, I made a mess with the sign-off again 🤦🏻

[kumar-mallikarjuna]

Hey @porridge , might need to fix your sign-off here 😅

[porridge]

Hey @porridge , might need to fix your sign-off here 😅

Yes, but unfortunately I'm not able to do it myself. GitHub allows me to push a commit to your repo's branch based on a review suggestion (which is what I did, but used the wrong email) but it will not allow me to rewrite that commit. Sorry but I'll have to ask you to squash that commit into yours.

[kumar-mallikarjuna]

Done, @porridge .