fix(examples): exit program if load_param, load_model failed for clear message

[zchrissirhcz]

No description provided.

[zchrissirhcz]

For #3982

Previously:

(base) ➜  examples git:(examples-early-stop-if-load-models-failed) ✗ ./yolox ./frontground.png
fopen yolox.param failed
fopen yolox.bin failed
find_blob_index_by_name images failed
Try
find_blob_index_by_name output failed
Try
[1]    40373 segmentation fault  ./yolox ./frontground.png

Now:

(base) ➜  examples git:(examples-early-stop-if-load-models-failed) ✗ ./yolox ./frontground.png 
fopen yolox.param failed

This may help people find error with precise message.

[nihui]

load function returns 0 if success
thus, the condition shall check any non-zero value for any potential failure

[zchrissirhcz]

Checking zero/nonzero now.
My rebase failed to push to origin, thus made an new commit.

[nihui]

Thanks for your contribution !