Check return value of file_put_contents

[gqqnbig]

https://github.com/gqqnbig-me/LatexOnWordPress/blob/e46c41092f4e6832ad8e9207df9c8e5299240271/wp-tex.php#L176

file_put_contents returns false if failed to write the file.

We may need to create folder first. https://stackoverflow.com/questions/13372179/creating-a-folder-when-i-run-file-put-contents