Gracefully handle project reference outside the workspace

[lacasseio]

Projects like GSCXScanner explicitly mention that the required project must be placed under the same parent. The general idea is both projects should be part of a bigger project, so reaching into an upper directory (outside the current project) would be allowed. However, reaching out would break the Gradle project mapping if the project is used as a standalone project. We should at least provide a better error message than the following:

* Where:
Settings file '<redacted>/GSCXScanner/settings.gradle' line: 9

* What went wrong:
An exception occurred applying plugin request [id: 'dev.nokee.xcode-build-adapter', version: '0.5.0']
> Failed to apply plugin 'dev.nokee.xcode-build-adapter'.
   > Xcode project '<redacted>/GSCXScanner/../GTXiLib/GTXiLib.xcodeproj' does not exists