centralize gdb command result class checking

each command can have only one of two legitimate responses: "error" or -
depending on the command, and thus declared via a flag - "done" or
"running".
this is way nicer than sprinkling the code with else-ifs (where elses
are sufficient) and asserts all over the place - and silently failing in
release builds.

src/plugins/debugger/gdb/gdbmi.h

@@ -160,6 +160,7 @@ class GdbResponse
 public:
     GdbResponse() : token(-1), resultClass(GdbResultUnknown) {}
     QByteArray toString() const;
+    static QByteArray stringFromResultClass(GdbResultClass resultClass);
 
     int            token;
     GdbResultClass resultClass;