Improve `graphman remove` feedback

[fordN]

Description

We should try to be explicit in the messages we return from graphman remove. i.e if a name is not found let the user now, if the user entered a string that looks like a namespace instead of a subgraph name nudge the user to not use namespaces, ....

Are you aware of any blockers that must be resolved before implementing this feature? If so, which? Link to any relevant GitHub issues.

Some information to help us out

• Tick this box if you plan on implementing this feature yourself.
• I have searched the issue tracker to make sure this issue is not a duplicate.