Refs #11054 Gracefully fail if dot unavailable

Code/Mantid/docs/runsphinx.py.in

@@ -51,7 +51,7 @@ def main(sysarg):
     if DIAGRAMS_DIR != "":
         os.environ["DIAGRAMS_DIR"] = DIAGRAMS_DIR
 
-    if DOT_EXECUTABLE != "":
+    if os.path.isfile(DOT_EXECUTABLE):
         os.environ["DOT_EXECUTABLE"] = DOT_EXECUTABLE
 
     # Arguments for main

Code/Mantid/docs/sphinxext/mantiddoc/directives/diagram.py

@@ -61,7 +61,9 @@ class DiagramDirective(BaseDirective):
         try:
             dot_executable = os.environ["DOT_EXECUTABLE"]
         except:
-            raise RuntimeError("The '.. diagram::' directive requires a DOT_EXECUTABLE environment variable to be set.")
+            self.add_rst(".. figure:: /images/ImageNotFound.png\n\n" +
+                        "    graphviz not found - diagram could not be rendered.")
+            return []
 
         #Make sure we have an output directory
         if not os.path.exists(out_dir):