detect and fail when replacement character (-I) is not used

[Ahmet Alp Balkan]

The other day I ran:

kubectl foreach -I _ [...bunch of filters...] -- get pods 

but I didn't realize I was getting the same result from all clusters (because I didn't use _ anywhere, and -I caused --context= from not being passed to the kubectl cmd).

Fail if nothing is replaced among args when used with -I and tell the user they specified -I sequence but never used it.