confirm-upgrade should provide user an error message when there is no pending upgrade to confirm.

[sangee2004]

Acorn version - v0.8.0-alpha7-200-gff70cc2b+ff70cc2b

Steps to reproduce the problem:
Scenario 1:

1. Deploy app with autoupgrade not enabled.
2. Try to update app with confirm-upgrade option.
   No update happens as expected. But there is no error message presented to the user and the output of the CLI indicates success which is misleading.

acorn update --confirm-upgrade mytest1 
mytest1

Scenario 2:

1. Deploy app with autoupgrade enabled.
2. Try to update app with confirm-upgrade option when there is no new version available.
   No update happens as expected. But there is no error message presented to the user and the output of the CLI indicates success which is misleading.

acorn update --confirm-upgrade mytest2 
mytest2

Another scenario were it gets confusing - #1726 (comment)