Handle errors more gracefully.

sphinx/environment.py

@@ -360,6 +360,7 @@ class BuildEnvironment:
                     continue
                 # finally, check the mtime of dependencies
                 for dep in self.dependencies.get(docname, ()):
+                    try:
                         deppath = path.join(self.srcdir, dep)
                         if not path.isfile(deppath):
                             changed.add(docname)
@@ -368,6 +369,10 @@ class BuildEnvironment:
                         if depmtime > mtime:
                             changed.add(docname)
                             break
+                    except EnvironmentError:
+                        # give it another chance
+                        changed.add(docname)
+                        break
 
         return added, changed, removed