Make missing public soname error message clear.

If a public library is missing, make it clear that there is no
library with that soname that is missing. This can help diagnose
problems if a library exists, but the library doesn't have the right
soname.

Bug: 29400363
(cherry picked from commit 9a84d90c)

Change-Id: Ie2306a2e28aff779d07441dc8af078256b184f8a