Unmet Dependencies reporting is poor

When loading configuration, for example that generated using features, if a dependency is not met the user is simply given an exception telling them which items had unmet dependencies. The actual unmet dependencies are not noted at all.

This appears to be a fundamental issue with the way that the code is structured in Drupal\Core\Config\ConfigInstaller because ConfigInstaller::validateDependencies simply returns TRUE or FALSE. Note that whilst it is reproducible using features the actual issue is in the core because features just uses ConfigInstaller::checkConfigurationToInstall().

It would appear that some combination of the following is in order: the failed dependencies need to be returned so the outer code can determine what to do with this information; there needs to be some mechanism to record this information for the outer code to look up the failed dependencies; a log message should be created (if possible at the time)