QGIS » QGIS Application

From a QGIS plugin, I call through Processing different Saga algs : "clipgridwithpolygon", "contourlinesfromgrid"

The alg "clipgridwithpolygon" accept unicode filename with non ascii char.
The alg contourlinesfromgrid does not.
The code runs fine with "non ascii filename" but when a non ascci appears:

STACK TRACE

"C:/Users/xx/.qgis2/python/plugins\\PhysiocapAnalyseur\\Physiocap_intra_interpolation.py", line 273, in physiocap_creer_raster_iso
​      
File "C:/Users/xx/.qgis2/python/plugins\\processing\\tools\\general.py", line 71, in runalg
alg = Processing.runAlgorithm(algOrName, None, *args, **kwargs)
File "C:/Users/xx/.qgis2/python/plugins\\processing\\core\\Processing.py", line 337, in runAlgorithm
print 'Unable to execute algorithm\
' + msg
UnicodeEncodeError: 'ascii' codec can't encode character u'\\xe9' in position 90: ordinal not in range(128)

The portion of code

from __future__ import unicode_literals
import procesing
....
raster_dans_poly = processing.runalg("saga:clipgridwithpolygon",
                nom_raster_temp,
                nom_vignette,
                nom_raster)

nom_raster = raster_dans_poly[ 'OUTPUT']​
​iso_dans_poly_brut = processing.runalg("saga:contourlinesfromgrid",
                    nom_raster,
                    isoMin, isoMax, isoInterlignes,
                    None)​  # here is line 273 but nom_raster is the problem

My Question : is this a limitation on the SAGA side or in Processing ?
Is it possible to raise Processing or SAGA exception ?

Problem experienced on Win8 64 Bits Qgis 2.12.3 Processing 2.12.2 Saga 2.1.2