Check if the java binary can be found before launch.

Fix #386

logic/MinecraftProcess.cpp

@@ -24,6 +24,7 @@
 #include <QDir>
 #include <QProcessEnvironment>
 #include <QRegularExpression>
+#include <QStandardPaths>
 
 #include "BaseInstance.h"
 
@@ -460,6 +461,14 @@ void MinecraftProcess::arm()
 	QString allArgs = args.join(", ");
 	emit log("Java Arguments:\n[" + censorPrivateInfo(allArgs) + "]\n\n");
 
+	auto realJavaPath = QStandardPaths::findExecutable(JavaPath);
+	if (realJavaPath.isEmpty())
+	{
+		emit log(tr("The java binary \"%1\" couldn't be found. You may have to set up java "
+					"if Minecraft fails to launch.").arg(JavaPath),
+				 MessageLevel::Warning);
+	}
+
 	// instantiate the launcher part
 	start(JavaPath, args);
 	if (!waitForStarted())