Handle unindexed images
such as :
Traceback (most recent call last):
File "./spip2pelican.py", line 651, in <module>
website.export_to_pelican()
File "./spip2pelican.py", line 608, in export_to_pelican
skip_reason = article.export_to_pelican()
File "./spip2pelican.py", line 382, in export_to_pelican
content = spip_to_markdown(self.text, self.website)
File "./spip2pelican.py", line 273, in spip_to_markdown
s = document(s, website)
File "./spip2pelican.py", line 224, in document
s = re.sub(r'<(doc|img)([0-9]+)\|.*>', doc_replace, s)
File "/Users/boileau/anaconda3/lib/python3.6/re.py", line 191, in sub
return _compile(pattern, flags).sub(repl, string, count)
File "./spip2pelican.py", line 217, in doc_replace
url = os.path.join(website.attachments_prefix, "IMG", website.doc_index[doc_id])
KeyError: 26
Ignore image and output a warning.