Add better error handling for when `airship install` is ran inside a non-flat package

[BSteffaniak]

PS C:\Users\Brade\GitHub\FlatWorkspace> airship install --use-links
0.30624724656096225
TypeError: Cannot read properties of undefined (reading 'getOrDefault')
    at Airship.getSourceForPackage (C:\Users\Brade\.flat\bin\.dist\airship\airship.js:685:35)
    at InstallService.installLocalPackage (C:\Users\Brade\.flat\bin\.dist\airship\airship.js:16289:29)
    at async InstallService.install1 (C:\Users\Brade\.flat\bin\.dist\airship\airship.js:16038:5)
    at async InstallService.install0 (C:\Users\Brade\.flat\bin\.dist\airship\airship.js:16018:5)
    at async Airship.run (C:\Users\Brade\.flat\bin\.dist\airship\airship.js:649:6)
    at async Airship.__chainAsync (C:\Users\Brade\.flat\bin\.dist\airship\airship.js:52:3)
    at async Airship.main (C:\Users\Brade\.flat\bin\.dist\airship\airship.js:277:5)