zmarkdown: fix Sentry error reporting

[StaloneLab]

Error reporting on Sentry is quite weird by now, it seems to report some things but not others. Currently, I would describe the behavior as almost useless because:

• we do not get any contextual information, so cannot help users encountering weird bugs;
• some errors might not be logged. At least, no errors were logged in a long time;
• the latest error which got thrown should not have been an error IMO, because invalid Markdown (table with two headers) was entered;
• warning are not logged, which leads to problems such as the one noticed in Passage à zmarkdown 11.3.0 zds-site#6410 (comment) (in French), that cannot be reproduced.

As a result of these problems, we get garbage when an error should in fact be silent and nothing when an error should be reported.
I am unsure of how the problem can be solved, but I will look at it for the upcoming version.