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

[mmd]

Quick and dirty test:

```
diff --git a/src/output-flex.cpp b/src/output-flex.cpp
index 3ac29fe..44bc5fe 100644
--- a/src/output-flex.cpp
+++ b/src/output-flex.cpp
@@ -983,6 +983,32 @@ void output_flex_t::add_row(table_connection_t *table_connection,
     }
 }
 
+static int msghandler (lua_State *L) {
+  const char *msg = lua_tostring(L, 1);
+  if (msg == NULL) {  /* is error object not a string? */
+    if (luaL_callmeta(L, 1, "__tostring") &&  /* does it have a metamethod */
+        lua_type(L, -1) == LUA_TSTRING)  /* that produces a string? */
+      return 1;  /* that is the message */
+    else
+      msg = lua_pushfstring(L, "(error object is a %s value)",
+                               luaL_typename(L, 1));
+  }
+  luaL_traceback(L, L, msg, 1);  /* append a standard traceback */
+  return 1;  /* return the traceback */
+}
+
+static int docall (lua_State *L, int narg, int nres) {
+  int status;
+  int base = lua_gettop(L) - narg;  /* function index */
+  lua_pushcfunction(L, msghandler);  /* push message handler */
+  lua_insert(L, base);  /* put it under function and args */
+  status = lua_pcall(L, narg, nres, base);
+  lua_remove(L, base);  /* remove message handler from the stack */
+  return status;
+}
+
+
+
 void output_flex_t::call_process_function(int index,
                                           osmium::OSMObject const &object)
 {
@@ -996,7 +1022,7 @@ void output_flex_t::call_process_function(int index,
         get_options()->extra_attributes); // the single argument
 
     luaX_set_context(lua_state(), this);
-    if (lua_pcall(lua_state(), 1, 0, 0)) {
+    if (docall(lua_state(), 1, 0)) {
         throw std::runtime_error{"Failed to execute lua processing function:"
                                  " {}"_format(lua_tostring(lua_state(), -1))};
     }
```

Returns:

```
Setting up table: planet_osm_rels
Using projection SRS 3857 (Spherical Mercator)

Reading in file: r1377803.osm
Using XML parser.
node cache: stored: 117(100.00%), storage efficiency: 1.39% (dense blocks: 1, sparse nodes: 116), hit rate: 100.00%
Osm2pgsql failed due to ERROR: Failed to execute lua processing function: /home/user/osm2pgsql/flex-config/compatible.lua:569: Error in 'add_row': Trying to add relation to table 'planet_osm_line'

stack traceback:
	[C]: in function 'add_row'
	/home/user/osm2pgsql/flex-config/compatible.lua:569: in function </home/user/osm2pgsql/flex-config/compatible.lua:534>
```

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