Always check the return values from the platform API

[erlingrj]

We have to go through our code base and make sure to always check the return values from the platform API.

lf_mutex_lock(env->mutex);

should be

if (lf_mutex_lock(env->mutex) != 0) {
  lf_print_error_and_exit("Failed to lock mutex"); 
};

[erlingrj]

In the enclaves2 branch I have added a macro called LF_ASSERT which you can pass a condition and it will invoke print and terminate if it is false. So we can do LF_ASSERT(lf_mutex_lock(env->mutex) == 0, "Failed to lock mutex"));. The assertions are disabled if LF_NOASSERT is defined.