cli: Requires -app flag if config set -scope=app is set.

[xiaolin-ninja]

Currently, if the scope is set explicitly as "app" without an app provided, we set the scope with an empty app. We should return a helpful error message instead.

Before:

❯ waypoint-dev config set -scope=app tester=test

❯ waypoint-dev config get
   SCOPE  |  NAME  | VALUE | WORKSPACE | LABELS  
----------+--------+-------+-----------+---------
  project | test   | test  |           |         
  app:    | tester | test  |           |        

After:

❯ waypoint-dev config set -scope=app testt=test
-scope requires -app set if scope is 'app'