toot-toot: gracefully handle lack of permissions

This opens up a whole new can of worms: Teliva's ability to show clear
error messages hinges on apps having decent error handling.

I suppose that's ok. A new app someone downloads that throws out strange
error messages and puts them in the equivalent of the console doesn't
exactly engender trust. It's reasonable to treat poor error handling as
breakage.

Here's my running list of open questions from working on the permissions
screen so far:
  - how to teach people to care about the difference between:
    - permissions known to be unsafe
    - permissions unknown to be safe
  - how to show syntax errors in the permissions screen
  - how to detect syntax errors in the permissions screen (lots of stuff
    only shows up when running)
  - how to deal with apps with poor error handling (this commit; punt for now)
  - how to engender skepticism in people towards what apps tell them
    ("oh, just go into the permissions screen and type ___ to fix this error")
  - how to help people judge the quality of their permissions
    (mode == 'r' vs mode ~= 'w')

toot-toot.tlv

@@ -312,8 +312,10 @@
     >    cursor = 1
     >  elseif key == 23 then  -- ctrl-w
     >    local out = io.open('toot', 'w')
-    >    out:write(prose, '\n')
-    >    out:close()
+    >    if out ~= nil then
+    >      out:write(prose, '\n')
+    >      out:close()
+    >    end
     >  elseif key == 10 or (key >= 32 and key < 127) then
     >    prose = prose:insert(string.char(key), cursor-1)
     >    cursor = cursor+1