src/gray_params.f90: handle file not found error

src/gray_params.f90

@@ -166,7 +166,7 @@ contains
     integer, intent(in), optional :: unit
 
     ! local variables
-    integer :: u
+    integer :: u, iostat
 
     if (present(unit)) then
       u=unit
@@ -174,6 +174,10 @@ contains
       u = get_free_unit()
     end if
     open(u,file=filenm,status='old',action='read',iostat=iostat)
+    if (iostat>0) then
+      print'(a)', 'gray_params file not found!'
+      call EXIT(1)
+    end if
 
     ! Raytracing
     ! ========================================================================