improve error handling for SourceRootDirs

Previously, warnings about missing modules were printed directly to
stderr. Instead we can pass these messages along as errors using the
existing pathways.

Bug: 269457150
Test: m nothing
Test: add -external to PRODUCT_SOURCE_ROOT_DIRS and observe missing
  module errors
Change-Id: I7273c427f38024e3c288f1ecb31175ed04ac44a6