don't throw an exception in the map thread for bad styles

test/api/set_style.cpp

@@ -0,0 +1,32 @@
+#include "../fixtures/util.hpp"
+#include "../fixtures/fixture_log_observer.hpp"
+
+#include <mbgl/map/map.hpp>
+#include <mbgl/platform/default/headless_view.hpp>
+#include <mbgl/platform/default/headless_display.hpp>
+#include <mbgl/storage/default_file_source.hpp>
+
+
+TEST(API, SetStyle) {
+    using namespace mbgl;
+
+    auto display = std::make_shared<mbgl::HeadlessDisplay>();
+    HeadlessView view(display);
+    DefaultFileSource fileSource(nullptr);
+
+    Log::setObserver(util::make_unique<FixtureLogObserver>());
+
+    Map map(view, fileSource);
+
+    map.start(Map::Mode::Static);
+
+    map.setStyleJSON("invalid", "test/suite");
+
+    map.stop();
+
+    auto observer = Log::removeObserver();
+    auto flo = dynamic_cast<FixtureLogObserver*>(observer.get());
+    EXPECT_EQ(1ul, flo->count({ EventSeverity::Error, Event::ParseStyle, -1,
+        "Error parsing style JSON at 0: Expect either an object or array at root" }));
+    EXPECT_TRUE(flo->unchecked().empty());
+}