linker: fix error message for inaccessible libs

Added a test to make sure linker produces correct error message
when user attempts to load a library in a linked namespace and fails.

Bug: http://b/67866190
Bug: http://b/64950640
Bug: http://b/64888291
Test: bionic-unit-test --gtest_filter=dlext*
Change-Id: I5b5c2070d1388eff123118350b2b5c8fc7571a29