Feature Request: Show warning during iOS build with missing .xcconfig

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Yes

Tell us about the problem

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letter.

Which platform(s) does your issue occur on?

iOS

Please provide the following version numbers that your issue occurs with:

• CLI: 4.2.0-2018-07-12-11970 (next)
  Project is using 4.1.0

Please tell us how to recreate the issue in as much detail as possible.

At some time in previous release, maybe NS 2 or 3x projects provided a build.xcconfig file which specifies:

ASSETCATALOG_COMPILER_APPICON_NAME = AppIcon;
ASSETCATALOG_COMPILER_LAUNCHIMAGE_NAME = LaunchImage;

for the app icons during a build which works great. However, if this file is deleted (happened on a project today and took hours to figure out why the icons weren't showing when installing the app), then the app icon is not set during a build and no warnings/info is provided.

This could be a potential time saver for others in the future if the file is not found. I don't know all the implications of that file during xcode builds so I won't pretend to 😄 - it would have saved me several hours tracking down the file was deleted months ago from the project repository and that I had to set the icons in xcode on the build settings page after opening the workspace.

Start by locating the iOS build handling in the CLI and search for references to build.xcconfig or the ASSETCATALOG_COMPILER settings. Reproduce a build with the file missing, then determine where a warning should appear and verify that the warning is shown without changing builds where the file exists.