netbpfload: fail if platform exec of apex fails

should never fail, since we now have M-2024-1 module prebuilts:
  $ adb shell ls -d /apex/com.android.tethering*
  /apex/com.android.tethering
  /apex/com.android.tethering@341310230

Test: TreeHugger, manually on a Pixel device
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: Ib145fba8e3c47bc300f4589723d9c80749770a19

netbpfload/NetBpfLoad.cpp

@@ -259,9 +259,11 @@ int main(int argc, char** argv, char * const envp[]) {
     }
 
     if (is_platform) {
+        ALOGI("Executing apex netbpfload...");
         const char * args[] = { apexNetBpfLoad, NULL, };
         execve(args[0], (char**)args, envp);
-        ALOGW("exec '%s' fail: %d[%s]", apexNetBpfLoad, errno, strerror(errno));
+        ALOGE("exec '%s' fail: %d[%s]", apexNetBpfLoad, errno, strerror(errno));
+        return 1;
     }
 
     if (!has_platform_bpfloader_rc && !has_platform_netbpfload_rc) {