Fix runtime error checking with lua* commands

"It's first result is the status code (a boolean), which is true if the call succeeds without errors. [...] In case of any error, pcall returns _**`false`**_ [not `nil`] plus the error message." -- Lua 5.1 manual

I also made some stylistic and commenting changes:
  * ~80 char comment/code wrap.
  * Capital letter starting comments.
  * Space between comment character(s) and first word.

But noticed that more are needed:
  * `func = function()` -> `function func()`.
  * Less commas in comment sentences.

You may or may not agree with all of these, but I find that they make the code much more readable and it's Minetest's code style.

Also, should probably be done but unrelated:
  * `sort_pos` copies positions, which is probably wastefull.
  * The module system needs work, because `code` depends on `manipulations` for `sort_pos`, maybe this should be in `init`.

**Edit:**
  * The `/` command namespace is far too generic, it's also easy to start typing builtin commands with the extra `/` or vice-versa.