Output formula errors in unit filters to WML error stream

(And consider them to match no units)

data/test/scenarios/filter_this_unit.cfg

@@ -59,7 +59,7 @@
 	[/event]
 )}
 
-{GENERIC_UNIT_TEST filter_this_unit_fai_short (
+{GENERIC_UNIT_TEST filter_fai_unit (
 	[event]
 		name=prestart
 		[modify_unit]
@@ -76,3 +76,15 @@
 		)}
 	[/event]
 )}
+
+{GENERIC_UNIT_TEST filter_fai_unit_error (
+	[event]
+		name=prestart
+		{RETURN (
+			[have_unit]
+				id=bob
+				formula="+ max_moves"
+			[/have_unit]
+		)}
+	[/event]
+)}

src/unit_formula_manager.cpp

@@ -19,17 +19,24 @@
 #include "formula.hpp"
 #include "formula_string_utils.hpp"
 #include "map_location.hpp"
+#include "log.hpp"
 
 #include <boost/foreach.hpp>
 
 bool unit_formula_manager::matches_filter(const std::string & cfg_formula, const map_location & loc, const unit & me)
 {
-	const unit_callable callable(loc,me);
-	const game_logic::formula form(cfg_formula);
-	if(!form.evaluate(callable).as_bool()) {///@todo use formula_ai
+	try {
+		const unit_callable callable(loc,me);
+		const game_logic::formula form(cfg_formula);
+		if(!form.evaluate(callable).as_bool()) {///@todo use formula_ai
+			return false;
+		}
+		return true;
+	} catch(game_logic::formula_error& e) {
+		lg::wml_error << "Formula error in unit filter: " << e.type << " at " << e.filename << ':' << e.line << ")\n";
+		// Formulae with syntax errors match nothing
 		return false;
 	}
-	return true;
 }
 
 void unit_formula_manager::add_formula_var(std::string str, variant var)

wml_test_schedule

@@ -147,7 +147,8 @@
 0 filter_this_unit_wml
 0 filter_this_unit_tl
 0 filter_this_unit_fai
-0 filter_this_unit_fai_short
+0 filter_fai_unit
+1 filter_fai_unit_error
 # Interrupt tag tests
 0 check_interrupts_break
 0 check_interrupts_return