Add Method to Parse Error Commands

This is a parsing method like the others, this time for error commands.

src/command.cpp

@@ -162,6 +162,23 @@ Command Command::parseStoreToken(const QJsonObject& input)
            );
 }
 
+Protocol::Command Protocol::Command::parseError(const QJsonObject& input)
+{
+    if (!input.keys().contains(QStringLiteral("message")) || !input.value(QStringLiteral("message")).isString()) {
+        logError(QStringLiteral("Error command does not contain message."));
+        return Command();
+    }
+    const QString token = (input.keys().contains(QStringLiteral("token")) && input.value(QStringLiteral("token")).isString())
+                          ? input.value(QStringLiteral("token")).toString()
+                          : QString();
+
+    return Command(
+               Action::error,
+               Arguments{std::make_pair(QStringLiteral("message"), input.value(QStringLiteral("message")).toString())},
+               token
+           );
+}
+
 QString Command::toString() const
 {
     if (action == Action::invalid)

src/command.h

@@ -71,6 +71,7 @@ private:
 
     static Command parseAuthenticate(const QJsonObject& input);
     static Command parseStoreToken(const QJsonObject& input);
+    static Command parseError(const QJsonObject& input);
 
     static QString getActionString(const Action action);
 };