Print error instead of failing silently when loading plugins

[pschmutz]

Previously the return value of dlopen was ignored, meaning a failed load of a shared library would stay unnoticed

Can be tested by compiling this piece of code into a shared library and then trying to load it

#include <iostream>
#include <mujoco/mujoco.h>

void test();

mjPLUGIN_LIB_INIT
{
  test();
  std::cout << "Registering Test Plugin" << std::endl;
}

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