df.new(): catch errors thrown by allocate()

This can include DFHack::Error::VTableMissing exceptions if the vtable of a
member field is unknown.

Fixes #3627

docs/changelog.txt

@@ -46,6 +46,7 @@ changelog.txt uses a syntax similar to RST, with a few special sequences:
 ## Internals
 
 ## Lua
+- ``new()``: improved error handling so that certain errors that were previously uncatchable (creating objects with members with unknown vtables) are now catchable with ``pcall()``
 
 ## Removed
 

library/LuaWrapper.cpp

@@ -685,7 +685,8 @@ static int meta_new(lua_State *state)
 
     type_identity *id = get_object_identity(state, 1, "df.new()", true);
 
-    void *ptr;
+    void *ptr = nullptr;
+    std::string err_context;
 
     // Support arrays of primitive types
     if (argc == 2)
@@ -703,11 +704,22 @@ static int meta_new(lua_State *state)
     }
     else
     {
-        ptr = id->allocate();
+        try {
+            ptr = id->allocate();
+        }
+        catch (std::exception &e) {
+            if (e.what()) {
+                err_context = e.what();
+            }
+        }
     }
 
     if (!ptr)
-        luaL_error(state, "Cannot allocate %s", id->getFullName().c_str());
+        luaL_error(state, "Cannot allocate %s%s%s",
+            id->getFullName().c_str(),
+            err_context.empty() ? "" : ": ",
+            err_context.c_str()
+        );
 
     if (lua_isuserdata(state, 1))
     {