error condition not properly documented for Porcelain.spawn

For some cases, Porcelain.spawn returns data of structure `{:error, }`. E.g:

```elixir
iex(23)> Porcelain.spawn("whoami2", [])
{:error, "Command not found: whoami2"}
```

But the spec doesn't capture it:

```elixir
@spec spawn(binary, [binary]) :: Porcelain.Process.t
@spec spawn(binary, [binary], Keyword.t) :: Porcelain.Process.t

def spawn(prog, args, options \\ [])
```

leading to little annoyances like warning messages about pattern matching with the error msg type.