Clearer error in case of Android.bp being unreadable

Bug: 64600838
Test: mkdir errtest \
      && ln -s /tmp/dontexist errtest/Android.bp \
      # and add errtest to ./Android.bp \
      && m nothing \
      # and check that the error message mentions a symlink

Change-Id: I841ec12d613f61ccc3396538062bee48c8c1ca27