Improve error message for pointerof

spec/compiler/semantic/pointer_spec.cr

@@ -63,11 +63,15 @@ describe "Semantic: pointer" do
   end
 
   it "pointer of class raises error" do
-    assert_error "pointerof(Int32)", "can't take address of Int32"
+    assert_error "pointerof(Int32)", "can't take address of Int32 because it's a Path. `pointerof` expects a variable or constant."
+  end
+
+  it "pointer of class raises error" do
+    assert_error "def foo; foo; end; pointerof(foo)", "can't take address of foo because it's a Call. `pointerof` expects a variable or constant."
   end
 
   it "pointer of value error" do
-    assert_error "pointerof(1)", "can't take address of 1"
+    assert_error "pointerof(1)", "can't take address of 1 because it's a NumberLiteral. `pointerof` expects a variable or constant."
   end
 
   it "types pointer value on typedef" do

src/compiler/crystal/semantic/main_visitor.cr

@@ -2589,7 +2589,7 @@ module Crystal
         # "undefined local variable or method"
         node.exp.accept self
 
-        node.exp.raise "can't take address of #{node.exp}"
+        node.exp.raise "can't take address of #{node.exp} because it's a #{ node.exp.class_desc }. `pointerof` expects a variable or constant."
       end
 
       node.bind_to var