Friendly error if entr isn't installed for build.sh

slides/build.sh

@@ -15,6 +15,12 @@ once)
   ;;
 
 forever)
+  # check if entr is installed
+  if ! command -v entr >/dev/null; then
+    echo >&2 "First install 'entr' with apt, brew, etc."
+    exit
+  fi
+  
   # There is a weird bug in entr, at least on MacOS,
   # where it doesn't restore the terminal to a clean
   # state when exitting. So let's try to work around