Print error message if system server jar is missing in prebuilt apex

If a system server jar declared in `PRODUCT_APEX_SYSTEM_SERVER_JAR` is
missing in the prebuilt apex file, a nil pointer exception is raised
during soong analysis. To improve debugagibity, replace this with an
error message that mentions the missing library name.

Test: presubmits
Bug: 388778489
Change-Id: Id01f8dd23b84e7735b7e45ebcd81fb1121f44ca5