[Tile-serving] [openstreetmap/osm2pgsql] Provide a stack trace on a lua error with flex backend (#1136)

[Paul Norman]

I don't care about the C++ backtrace, I want the lua stack traceback. I have no idea what function add_row was called from.

The lua interpreter's stack traceback is

```
lua: ./openstreetmap-carto.lua:555: bad argument #1 to 'ipairs' (table expected, got nil)
stack traceback:
        [C]: in function 'ipairs'
        ./openstreetmap-carto.lua:555: in function 'add_route'
        ./openstreetmap-carto.lua:610: in function 'process_relation'
        test.lua:316: in main chunk
        [C]: in ?
```

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