feat: improve install error when app doesnt exist

[rgee0]

Description

When trying to install an app that doesnt exist the error message would advise the user that the app doesnt exist and that they should run install --help to see what is available.

However, there may be situations where the user is using the wrong verb because they are trying to get a tool, rather than an app. This change introduces a check when an app not found error is encountered to see whether a tool with the user provided name exists and advises them accordingly.

also adds associated test.

Motivation and Context

• I have raised an issue to propose this change, which has been given a label of design/approved by a maintainer (required)
  discussed on the community call on 20/11

How Has This Been Tested?

Functional

Tried the helm example with isnt an app but will resolve to a tool. Then tried burt which is neither app nor tool.

➜  arkade git:(addGetPrompt) make build                                                         
go build
➜  arkade git:(addGetPrompt) ./arkade install helm                                              
Error: no such app. helm is available as a tool, run "arkade get helm" to get it
➜  arkade git:(addGetPrompt) ./arkade install burt                                              
Error: no such app: burt, run "arkade install --help" for a list of apps

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 --format 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 tested this on arm, or have added code to prevent deployment