后半部分的好多图片下载失败，是不是wkhtmltopdf分配的缓存太小了

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周半部分的好多图片下载失败，是不是wkhtmltopdf分配的缓存太小了。因为失败的图片总是后半部分的图。而错误信息也没有提示什么有用的：
Traceback (most recent call last):
File "crawler.py", line 165, in
crawler.run()
File "crawler.py", line 99, in run
pdfkit.from_file(htmls, self.name + ".pdf", options=options)
File "D:\Program Files\Python36\lib\site-packages\pdfkit\api.py", line 49, in from_file
return r.to_pdf(output_path)
File "D:\Program Files\Python36\lib\site-packages\pdfkit\pdfkit.py", line 156, in to_pdf
raise IOError('wkhtmltopdf reported an error:\n' + stderr)
OSError: wkhtmltopdf reported an error:
Loading pages (1/6)
Warning: Failed to load file:///static/img/404.png (ignore)
Counting pages (2/6)
Resolving links (4/6)
Loading headers and footers (5/6)
Printing pages (6/6)
Done
Exit with code 1 due to network error: ProtocolUnknownError