Small UX improvements to some `src extensions` commands

[Kelvin-Lee]

1. Can we get the src extensions copy command to print "Extension copied!", instead of "Extension published"? (call to print in extensions_copy.go)
2. Can we get src extensions delete to detect if the -id flag is not being passed? (extensions_delete.go)Today, src extensions delete <valid graphql id> will report:

GraphQL errors: 1 error occurred:
	* {
  "message": "invalid graphql.ID",
  "path": [
    "extensionRegistry",
    "deleteExtension"
  ]
}

i.e. it's explicitly expecting the -id flag, I'm pretty sure.

3. Can we get some sort of src <where am i> , to get feedback of which SRC_ENDPOINT we're connected to? I couldn't find an obvious method/currently existing command that does this. Maybe like... src where or something? src info? Would be a nice to have for me personally since support team sometimes has to connect to different instances to test things out.