Actually fix the race condition. Can't do any initialization step while waiting.

Core/System.cpp

@@ -477,11 +477,6 @@ bool PSP_InitStart(const CoreParameter &coreParam, std::string *error_string) {
 }
 
 bool PSP_InitUpdate(std::string *error_string) {
-	if (Achievements::IsBlockingExecution()) {
-		// Keep waiting.
-		return false;
-	}
-
 	if (pspIsInited || !pspIsIniting) {
 		return true;
 	}
@@ -491,7 +486,10 @@ bool PSP_InitUpdate(std::string *error_string) {
 	}
 
 	bool success = !g_CoreParameter.fileToStart.empty();
-	*error_string = g_CoreParameter.errorString;
+	if (!g_CoreParameter.errorString.empty()) {
+		*error_string = g_CoreParameter.errorString;
+	}
+
 	if (success && gpu == nullptr) {
 		PSP_SetLoading("Starting graphics...");
 		Draw::DrawContext *draw = g_CoreParameter.graphicsContext ? g_CoreParameter.graphicsContext->GetDrawContext() : nullptr;

UI/EmuScreen.cpp

@@ -233,6 +233,11 @@ bool EmuScreen::bootAllowStorage(const Path &filename) {
 void EmuScreen::bootGame(const Path &filename) {
 	auto sc = GetI18NCategory(I18NCat::SCREEN);
 
+	if (Achievements::IsBlockingExecution()) {
+		// Keep waiting.
+		return;
+	}
+
 	if (PSP_IsRebooting())
 		return;
 	if (PSP_IsInited()) {
@@ -244,6 +249,7 @@ void EmuScreen::bootGame(const Path &filename) {
 
 	if (PSP_IsIniting()) {
 		std::string error_string = "(unknown error)";
+
 		bootPending_ = !PSP_InitUpdate(&error_string);
 
 		if (!bootPending_) {