When unknown activity is used to create a new entry, default project&service is used

[roughy]

• Version: 0.1.3
• Summary: Using the activity feature to create a new entry with an unknown activity mite-go create a new entry with my default project and service.
• Steps to reproduce:
  1. Configure default project & service
  2. Configure an activity foo
  3. go run main.go entries create -a bar -d 1h -n "baz"
• Expected result: Error message telling me that the activity bar is not configured
• Actual result: New entry is created with default project & service
• Nice to have: Show me a list of available activities in such a case.