linker: log only real dlerrors

Currently linker reports all potential dlerrors
for example if library was not found in 'this'
namespace - it initializes linker error buffer
with not found message but when consequent
search in linked namespace succeeds this message
is droped because dlopen was successful.

This commit avoids logging false positive error
messages when debug.ld.* set to dlerror.

Test: manual
Change-Id: I480694a1b1bbacd6bd1d8505cd2ee491710964cc