[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.

src/mbgl/style/parser.cpp

@@ -14,24 +14,27 @@
 
 #include <algorithm>
 #include <set>
+#include <sstream>
 
 namespace mbgl {
 namespace style {
 
 Parser::~Parser() = default;
 
-void Parser::parse(const std::string& json) {
+StyleParseResult Parser::parse(const std::string& json) {
     rapidjson::GenericDocument<rapidjson::UTF8<>, rapidjson::CrtAllocator> document;
     document.Parse<0>(json.c_str());
 
     if (document.HasParseError()) {
-        Log::Error(Event::ParseStyle, "Error parsing style JSON at %i: %s", document.GetErrorOffset(), rapidjson::GetParseError_En(document.GetParseError()));
-        return;
+        std::stringstream message;
+        message <<  document.GetErrorOffset() << " - "
+            << rapidjson::GetParseError_En(document.GetParseError());
+
+        return std::make_exception_ptr(std::runtime_error(message.str()));
     }
 
     if (!document.IsObject()) {
-        Log::Error(Event::ParseStyle, "Style JSON must be an object");
-        return;
+        return std::make_exception_ptr(std::runtime_error("style must be an object"));
     }
 
     if (document.HasMember("version")) {
@@ -100,6 +103,8 @@ void Parser::parse(const std::string& json) {
             glyphURL = { glyphs.GetString(), glyphs.GetStringLength() };
         }
     }
+
+    return nullptr;
 }
 
 void Parser::parseSources(const JSValue& value) {