fix exception raising in svg2pdf

IPython/nbconvert/transformers/svg2pdf.py

@@ -91,4 +91,4 b' class SVG2PDFTransformer(ConvertFiguresTransformer):'
                     # PDF is a nb supported binary, data type, so base64 encode.
                     return base64.encodestring(f.read())
             else:
-                return TypeError("Inkscape svg to png conversion failed")
+                raise TypeError("Inkscape svg to png conversion failed")