Add fatal failure checking method to Backend

The method allows the caller to check whether the backend has suffered a
fatal failure.

include/maxscale/backend.hh

@@ -182,6 +182,17 @@ public:
      */
     bool is_closed() const;
 
+    /**
+     * @brief Check if the backend has failed fatally
+     *
+     * When a fatal failure occurs in a backend, the backend server can no longer
+     * be used by this session. Fatal failures can occur when the execution of
+     * a session command fails on the backend but the expected result is different.
+     *
+     * @return True if a fatal failure has occurred in the backend server
+     */
+    bool has_failed() const;
+
 private:
     /**
      * Internal state of the backend

server/core/backend.cc

@@ -241,3 +241,8 @@ bool Backend::is_closed() const
 {
     return m_state & CLOSED;
 }
+
+bool Backend::has_failed() const
+{
+    return m_state & FATAL_FAILURE;
+}