Force errors when building docs to return non-0 exit code

docs/Makefile

@@ -3,7 +3,7 @@
 
 # You can set these variables from the command line, and also
 # from the environment for the first two.
-SPHINXOPTS    ?=
+SPHINXOPTS    ?= -W --keep-going
 SPHINXBUILD   ?= sphinx-build
 SOURCEDIR     = .
 BUILDDIR      = _build

docs/conf.py

@@ -103,7 +103,14 @@
 # List of patterns, relative to source directory, that match files and
 # directories to ignore when looking for source files.
 # This patterns also effect to html_static_path and html_extra_path
-exclude_patterns = ["_build", "Thumbs.db", ".DS_Store"]
+exclude_patterns = [
+    "_build",
+    "Thumbs.db",
+    ".DS_Store",
+    # We want to include images in drawio directory, but not the documentation for
+    # updating those images.
+    "drawio/**",
+]
 
 # The reST default role (used for this markup: `text`) to use for all
 # documents.