Enum fields must be required or have a default.

This is a breaking change, but the previous behaviour was broken so I'm
not concerned.

Also made most programmer errors more useful by giving type.field
context information.

Fixes #179.

tag_test.go

@@ -198,5 +198,5 @@ func TestInvalidRuneErrors(t *testing.T) {
 		Flag bool `short:"invalid"`
 	}{}
 	_, err := kong.New(&cli)
-	require.EqualError(t, err, "invalid short flag name \"invalid\": invalid rune")
+	require.EqualError(t, err, "<anonymous struct>.Flag: invalid short flag name \"invalid\": invalid rune")
 }