Exception inside contextmanager should cancel the zim creation

Start at the Python writer.Creator context-manager entry point and reproduce the issue with the example in the report. Trace what happens when the block raises an exception; done means creation is cancelled, temporary files are removed, and no incomplete valid ZIM file remains.

When using the context-manager, should a (non libzim) error occur, the exception is raised but the finalization is done on the Creator as if everything went well.

with libzim.writer.Creator(
    "test_x07.zim", main_page="A/index.html", index_language="eng", min_chunk_size=2048,
) as zfile:
    zfile.add_article(DumbArticle("index.html", "hello", ARTICLE_MIME, "bonjour"))
    raise Exception("outch")
    zfile.add_article(DumbArticle("page2.html", "hello2", ARTICLE_MIME, "bonjour2"))

T:0; A:4; RA:0; CA:4; UA:0; FA:0; IA:1; C:0; CC:0; UC:0; WC:1
T:0; Waiting for workers
T:0; ResolveRedirectIndexes
Resolve redirect
T:0; Set article indexes
set index
T:0; Resolve mimetype
T:0; create title index
T:0; 6 title index created
T:0; 2 clusters created
T:0; write zimfile :
T:0;  write mimetype list
T:0;  write directory entries
T:0;  write url prt list
T:0;  write title index
T:0;  write cluster offset list
T:0;  write header
T:0;  write checksum
T:0; rename tmpfile to final one.
T:0; finish
Traceback (most recent call last):
  File "./demo.py", line 55, in <module>
    raise Exception("outch")
Exception: outch

This results in a valid ZIM file on the filesystem but lacking the second article of course.

I think the expected behavior would be to cancel the ZIM creation and remove temporary files.

@mgautierfr @kelson42 ?