[Tile-serving] [openstreetmap/osm2pgsql] Give a better error when a transform returns a non-string type (#664)

[Paul Norman]

I had a multi backend transform which returned tags `{foo=true}`, and it reports the error

```
terminate called after throwing an instance of 'std::logic_error'
  what():  basic_string::_M_construct null not valid
```

This error doesn't give any clue that the problem is in the transform

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