Plugin: do not print out error messages if a symbol is not found
Plugin: do not print out error messages if a symbol is not found

The plugins interface try to load every known symbol from a loaded
plugin, not all plugins implements all of them so when dlerror()
returns NULL, it should not print errors to stdout.

Signed-off-by: Eduardo Silva <b6525c140147034c280e3cd2f39161f32f3f4f62@gmail.com>

sujayraaj/monkey_rtems,sujayraaj/monkey_rtems,monkey/monkey,dougsko/monkey,sujayraaj/monkeyTest,dougsko/monkey,dreamsxin/monkey,sujayraaj/monkey,dreamsxin/monkey,monkey/monkey,sujayraaj/monkey_rtems,WilliamRen/monkey-1,sujayraaj/monkey,dougsko/monkey,WilliamRen/monkey-1,sujayraaj/monkey_rtems,sujayraaj/monkey,sbagmeijer/monkey,sujayraaj/monkeyTest,sujayraaj/monkey_rtems,monkey/monkey,WilliamRen/monkey-1,monkey/monkey,sujayraaj/monkeyTest,dreamsxin/monkey,WilliamRen/monkey-1,dougsko/monkey,sbagmeijer/monkey,sbagmeijer/monkey,sbagmeijer/monkey,dougsko/monkey,dreamsxin/monkey,WilliamRen/monkey-1,sujayraaj/monkeyTest,monkey/monkey,sujayraaj/monkey,sbagmeijer/monkey,dreamsxin/monkey,monkey/monkey,sujayraaj/monkey