prepdata.ps1 is not doing proper error handling

> Please provide us with the following information:
> ---------------------------------------------------------------

### This issue is for a: (mark with an `x`)
```
- [x ] bug report -> please search issues before submitting
- [ ] feature request
- [ ] documentation issue or request
- [ ] regression (a behavior that used to work and stopped in a new release)
```

### Minimal steps to reproduce
> run `azd up` against the AOAISearchDemo code. when there are failures you are not bubbling up the failure.

### Any log messages given by the failure
>

Look at Line 65...this is but one example...it's everywhere in that script...

if ($process.ExitCode -ne 0) {
Write-Host ""
Write-Warning "Installing post-deployment dependencies failed with non-zero exit code $LastExitCode."
Write-Host ""
exit $process.ExitCode
}

You are printing 0 basically, even when there is a failure.

### Expected/desired behavior
>should throw the correct error

### OS and Version?
> Windows 7, 8 or 10. Linux (which distribution). macOS (Yosemite? El Capitan? Sierra?)

### Versions
>

### Mention any other details that might be useful

> ---------------------------------------------------------------
> Thanks! We'll be in touch soon.