Add error id in gtCrystGetSymmetryOperators

Signed-off-by: Yoann Guilhem <yoann.guilhem@esrf.fr>

zUtil_Cryst/gtCrystGetSymmetryOperators.m

@@ -340,7 +340,8 @@ case 'trigonal'
         symm(6).g3 = symm(3).g3 * symm(4).g3 * symm(2).g3;
 
     otherwise
-        gtError(['Unsupported spacegroup: ', num2str(spacegroup)]);
+        gtError('gtCrystGetSymmetryOperators:wrong_spacegroup', ...
+            ['Unsupported spacegroup: ', num2str(spacegroup)]);
     end
     
 end % end switch crystal_system