Throw a Lua exception when creating a negative-HP unit in a Lua context

Instead of throwing a WML error. This allows the UMC author to get a stack
trace if the unit creation was triggered from Lua.

Requested by @gfgtdf in a comment of #3042.

(cherry-picked from commit 258a0e4)

src/game_events/pump.cpp

@@ -647,6 +647,11 @@ std::size_t wml_event_pump::wml_tracking()
 	return impl_->internal_wml_tracking;
 }
 
+bool wml_event_pump::running() const
+{
+	return impl_->instance_count > 0u;
+}
+
 wml_event_pump::wml_event_pump(manager& man)
 	: impl_(new pump_impl(man))
 {

src/game_events/pump.hpp

@@ -144,6 +144,9 @@ class wml_event_pump
 	/** This function can be used to detect when no WML/Lua has been executed. */
 	std::size_t wml_tracking();
 
+	/** Returns true if we're currently executing an event handler. */
+	bool running() const;
+
 private:
 	bool filter_event(const event_handler& handler, const queued_event& ev);
 

src/units/unit.cpp

@@ -22,13 +22,15 @@
 #include "color.hpp"
 #include "deprecation.hpp"
 #include "display.hpp"
+#include "filter_context.hpp"
 #include "formatter.hpp"
 #include "formula/string_utils.hpp" // for VGETTEXT
 #include "game_board.hpp"			// for game_board
 #include "game_config.hpp"			// for add_color_info, etc
 #include "game_data.hpp"
 #include "game_errors.hpp"		   // for game_error
-#include "game_events/manager.hpp" // for add_events
+#include "game_events/manager.hpp" // for add_events, pump
+#include "game_events/pump.hpp" // for running
 #include "preferences/game.hpp"	// for encountered_units
 #include "gettext.hpp"			   // for N_
 #include "lexical_cast.hpp"
@@ -55,6 +57,7 @@
 #include "utils/functional.hpp"
 #include <boost/dynamic_bitset.hpp>
 #include <boost/function_output_iterator.hpp>
+#include "lua/lauxlib.h"
 
 #ifdef _MSC_VER
 #pragma warning (push)
@@ -632,8 +635,13 @@ void unit::init(const config& cfg, bool use_traits, const vconfig* vcfg)
 	}
 
 	if(const config::attribute_value* v = cfg.get("hitpoints")) {
-		if(loc_ != dying_unit_loc) {
-			VALIDATE(*v > 0, _("Unit with negative HP found"));
+		if(!(*v > 0 || loc_ == dying_unit_loc)) {
+			if(resources::game_events->pump().running()) {
+				// This function throws an exception, it doesn't return
+				luaL_error(resources::filter_con->get_lua_kernel()->get_state(), "Attempted to create a unit with negative HP");
+			} else {
+				VALIDATE(false, _("Unit with negative HP found"));
+			}
 		}
 		hit_points_ = *v;
 	} else {