reverts e0d92b9cc2#28502
Problem:
`vim.ui.open()` has a `pcall()` like signature, under the assumption
that this is the Lua idiom for returning result-or-error. However, the
`result|nil, errmsg|nil` pattern:
- has precedent in:
- `io.open`
- `vim.uv` (`:help luv-error-handling`)
- has these advantages:
- Can be used with `assert()`:
```
local result, err = assert(foobar())
```
- Allows LuaLS to infer the type of `result`:
```
local result, err = foobar()
if err then
...
elseif result then
...
end
```
Solution:
- Revert to the `result|nil, errmsg|nil` pattern.
- Document the pattern in our guidelines.
40ce857797