Avoid stopping build in presence of failing gallery examples in serve mode

[smarie]

When used with sphinx-autobuild, sphinx-gallery seems to avoid crashing in presence of failing examples. Instead one can browse the failing page, hopefully fix the problem in the gallery example, and watch the autobuild rebuild the fixed page.

We can't unfortunately do this today with mkdocs-gallery: mkdocs serve will be stopped by mkdocs-gallery's exception when a gallery example fails.

Proposal: maybe have a look at sphinx-gallery's code to make sure that there is actually an explicit code for this. If so, just copy it here. Otherwise, Let's add a try/except block when we detect that mkdocs runs in serve mode.