LibGemini: Do not loop in Gemini::Job in case of error

Previously, the job was shutdown only on a deferred invoke while there
was still data to be read. This meant that the read callback would get
called again and again, and, potentially throw the error again and again

This patch introoduces a failed state for the protocol parser and
returns early from the read callback if it has already failed

Userland/Libraries/LibGemini/Job.h

@@ -46,6 +46,7 @@ protected:
         InStatus,
         InBody,
         Finished,
+        Failed,
     };
 
     GeminiRequest m_request;