Spelling mistakes in TraceConnect are not handled

When connecting to a trace source, a spelling error in the name does not end in a simulation error or warning, but the simulation runs without hooking anything.

Example:

Ptr<SomeObject> a = ....;

a->TraceConnectWithoutContext ("Txx", MakeCallback(&SomeTraceSink)); // The real trace source name is 'Tx'

The code for TraceConnectWithoutContext is:

bool
ObjectBase::TraceConnectWithoutContext(std::string name, const CallbackBase& cb)
{
    NS_LOG_FUNCTION(this << name << &cb);
    TypeId tid = GetInstanceTypeId();
    Ptr<const TraceSourceAccessor> accessor = tid.LookupTraceSourceByName(name);
    if (!accessor)
    {
        return false;
    }
    bool ok = accessor->ConnectWithoutContext(this, cb);
    return ok;
}

If we enter in the if, we return false, but if that's a real mistake, then it would be more safe to abort the simulation with a meaningful error message.