Handle Undeploy for UI only apps

[sandeep-paliwal]

AIO scripts will now throw error when undeploy actions is called but there is no backend.
Handle this gracefully in undeploy command and not call scripts undeploy unless we have a valid backend

Description

Related Issue

Motivation and Context

How Has This Been Tested?

Screenshots (if appropriate):

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)

Checklist:

• [x ] I have signed the Adobe Open Source CLA.
• [x ] My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• [ x] I have read the CONTRIBUTING document.
• [x ] I have added tests to cover my changes.
• All new and existing tests passed.

[codecov]

Codecov Report

Merging #170 into master will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #170      +/-   ##
==========================================
+ Coverage   93.78%   93.81%   +0.03%     
==========================================
  Files          20       20              
  Lines         563      566       +3     
  Branches       67       68       +1     
==========================================
+ Hits          528      531       +3     
  Misses         31       31              
  Partials        4        4              

Impacted Files	Coverage Δ
src/commands/app/undeploy.js	100.00% <100.00%> (ø)

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[moritzraho]

@sandeep-paliwal can we change this to fs.existsSync('manifest.yml') so we are aligned with app-scripts and #171