meshpy reports gmsh failure when warnings occur

The meshpy error processing is a bit too sensitive in that it simply scans for the word 'error' in the stderr of the gmsh output (https://github.com/inducer/meshpy/blob/master/meshpy/gmsh.py#L182). This is problematic if some warning occurs during the gmsh run, which typically leads to output like this:

> Warning : ------------------------------
> Warning : Mesh generation error summary
> Warning : 6 warnings
> Warning : 0 errors
> Warning : Check the full log for details
> Warning : ------------------------------

Clearly, there was no error, but meshpy still throws a `GmshError`.