refactor(angular, react, vue): add missing test app msg (#28951)

Issue number: N/A

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## What is the current behavior?
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The packages, Angular, React, and Vue, will accept any `app_dir`
parameter when passing it to the build script. This can lead to a user
to entering a directory that doesn't exist within `test/apps`. If this
happens, then the build folder will include an incomplete test app.

For example:
1. `./build ng13`
2. This creates a `build/ng13` folder, but the folder only contains the
contents from the `base` folder. This causes it to be incomplete.

## What is the new behavior?
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this PR. -->

- Provides a message that the provided app directory doesn't exist.
- Does not create the incomplete app when an invalid app directory is
given.

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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## Other information

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screenshots of how the component looks before and after the change. -->

N/A