fix error message in Material.__init__()
fix error message in Material.__init__()

vlukes/sfepy,sfepy/sfepy,vlukes/sfepy,sfepy/sfepy,rc/sfepy,vlukes/sfepy,rc/sfepy,BubuLK/sfepy,BubuLK/sfepy,rc/sfepy,sfepy/sfepy,BubuLK/sfepy