Return an error in case of an unknown app to install command

[YuviGold]

Description

Return an error in case of an unknown app given to install command.
Exit with an error and appropriate message.
The SilenceUsage was turned on so the error message would be clear, just like get command.
In case the user wants to see the help message, he can ask for it explicitly.

Motivation and Context

• I have raised an issue to propose this change (required)
  No error message for failed install command #525

How Has This Been Tested?

➜  arkade git:(install-unknown-app) ./arkade install kind
Error: cannot install app: kind
➜  arkade git:(install-unknown-app) echo $?
1

Are you a GitHub Sponsor (Yes/No?)

• Yes
• No

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Documentation

• I have updated the list of tools in README.md if (required) with ./arkade get -o markdown
• I have updated the list of apps in README.md if (required) with ./arkade install --help

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have signed-off my commits with git commit -s
• I have added tests to cover my changes.
• All new and existing tests passed.

• I have tested this on arm, or have added code to prevent deployment

[alexellis]

Approved