Fix some corner cases in PathTracer

1) The "Starting page load" message is now only shown when path-trace-report is set to all.
   Previously, the intention had been to show this header always and only include the function/bytecode calls when the 'all' setting was used, but this seems a bit inconsistent (and it was not implemented correctly!).
2) If a trace messaage is received before any trace starts, then this is an error.