Refine error messages in case of missing api files

Some non-interpretable error messages has been shown in case of missing
current api files. Instead of that messy errors, This clearly tells
users what is the problem and how to fix it.

Bug: 126259114
Test: 1) try to build a sysprop_library and a java_sdk_library without
creating txt files under api/, and see it fails.
Test: 2) Copy and paste the emitted command, and try to build again.

Change-Id: I986781cc710475647c2b996b0f1c51a6f50c0ba2