Print an error for nonexisting redeclared modifier elements in extends.

[Per Östlund]

We should print an error if a redeclared modifier element can't be found in an extended class, such as this:

model M
end M;

model NonexistentRedeclareModifier2
  extends M(redeclare Real x);
end NonexistentRedeclareModifier2;

But the dependency analysis thinks that the redeclare modifier is unused, so it just removes it.