Fixed utils.execute returning different values for Lua 5.1 and Lua 5.2

[The documentation](http://stevedonovan.github.io/Penlight/api/modules/pl.utils.html#execute) states that `utils.execute` returns the same for Lua 5.1 and Lua 5.2, but it currently doesn't: on failure it returns `false` + error code for Lua 5.1, but `nil` + error code for Lua 5.2. This fix makes it return `false` in both cases.