graphviz errors are reported only once and obsolete graphs are left behind

### Describe the bug Error handling is generally not tested in the software industry and `sphinx.ext.graphviz` is no exception :-( When graphviz' `dot` tool fails to generate a diagram embedded with `.. graphviz ::` due to some error in the embedded diagram source code, the error is reported only once. If you invoke `sphinx-build` again then the error disappears. That's because the `foo.html` output file does not care about graphviz errors and is generated "incomplete" (i.e.: pointing to a missing .png file) anyway. So, the source `foo.rst` file is not processed again because it's output is already present and deemed OK. Obvious fix 1: stop ignoring graphviz failures and don't generate incomplete .html output. Or generate it in some non-final location to help with debugging but don't put it in the final location. To make things much worse, `sphinx.ext.graphviz` never deletes old `html/_images/graphviz-e8ab96ded74ad33e0d9517bcb46629283b059273.png` files. So, if you had successfully built an older and working version of the embedded `.. graphviz ::` diagram in the past and then you make a graphviz mistake, then you will get the last successful version of the `.png` file instead of a blank one. This makes it even more likely to miss errors: a missing diagram is relatively easy to notice while an older version is much easier to miss. Obvious fix 2: before trying to generate any new `.png` file (or any generated file anywhere really), start by deleting all the obsolete ones with the same filenames. ### How to Reproduce conf.py ``` extensions = [ "sphinx.ext.graphviz", ] ``` index.rst ``` This is a graphiz test ###################### .. graphviz:: :caption: Flat state machine diagram1 digraph smf_flat { node [style=rounded SYNTAX_ERROR=<<<<]; init [shape = point]; STATE_S0 [shape = box]; STATE_S1 [shape = box]; STATE_S2 [shape = box]; init -> STATE_S0; STATE_S0 -> STATE_S1; STATE_S1 -> STATE_S2; STATE_S2 -> STATE_S0; } ``` ### Environment Information ```text Platform: linux; (Linux-6.19.9-200.fc43.x86_64-x86_64-with-glibc2.42) Python version: 3.14.3 (main, Feb 4 2026, 00:00:00) [GCC 15.2.1 20260123 (Red Hat 15.2.1-7)]) Python implementation: CPython Sphinx version: 9.1.0 Docutils version: 0.21.2 Jinja2 version: 3.1.6 Pygments version: 2.19.1 ``` ### Sphinx extensions ```python `sphinx.ext.graphviz` ``` ### Additional context This was originally found and described in - https://github.com/zephyrproject-rtos/zephyr/issues/99932