[core] Emit MapChangeDidFailLoadingMap when the style cannot be loaded or parsed

Currently this signal is never emitted, which can cause the Still mode
to starve in case of an invalid style or failed request.

test/style/style_parser.cpp

@@ -4,6 +4,7 @@
 #include <mbgl/style/parser.hpp>
 #include <mbgl/util/io.hpp>
 #include <mbgl/util/enum.hpp>
+#include <mbgl/util/string.hpp>
 #include <mbgl/util/tileset.hpp>
 
 #include <rapidjson/document.h>
@@ -32,7 +33,11 @@ TEST_P(StyleParserTest, ParseStyle) {
     Log::setObserver(std::unique_ptr<Log::Observer>(observer));
 
     style::Parser parser;
-    parser.parse(util::read_file(base + ".style.json"));
+    auto error = parser.parse(util::read_file(base + ".style.json"));
+
+    if (error) {
+        Log::Error(Event::ParseStyle, "Failed to parse style: %s", util::toString(error).c_str());
+    }
 
     for (auto it = infoDoc.MemberBegin(), end = infoDoc.MemberEnd(); it != end; it++) {
         const std::string name { it->name.GetString(), it->name.GetStringLength() };