Fix bug 288164: don't crash if GraphViz binary not found

The bug only showed up on Windows, but could have happened
on Linux as well. This actually fixes 2 bugs:
* if it is detected that calling 'dot' fails before data
is written to the child process, a 0-deref SEGFAULT happens.
Now we use a local copy of the QProcess variable which is
not set to zero on error.
* with first bug fixed, we could show wrong message, as
the timer about warning on long layouting eventually
is started only after the error message is shown. This
error therefore soon is overwritting by misleading warning.