emit all error messages about invalid dependencies at once

I find it a bit annoying when the number of dependencies is large, and many of them were installed from local source, because I will have to repeat the cycle of "deploy -> fail -> install -> redeploy -> fail again -> install...". Therefore I prefer `createAppManifest()` telling me all the unqualified packages at once.

Now the error message will be like this:

```
Preparing to deploy application...DONE
Uploading application bundle...Error: 
* Unable to deploy package dependency 'shiny'
  
  The package was installed locally from source. Only packages
  installed from CRAN or GitHub are supported.

* Unable to deploy package dependency 'formatR'
  
  The package was installed locally from source. Only packages
  installed from CRAN or GitHub are supported.
Execution halted
```