[patch] [CLI] Add assertError and tiny fix

[wolfram]

1) This patch adds assertError() so you can also test for certain errors.

t.assertError(dojox.data.QueryReadStore.InvalidAttributeError, store, "getValue", [item, "NOT THERE"]);
t.assertError(dojox.data.QueryReadStore.InvalidItemError, store, "getValue", ["not an item", "NOT THERE"]);

2) fixed a comparison for undefined, use === for it

[wolfram]

add assertError()

[alex]

(In [10976]) merging patch from Wolfram to add an assertError to check for typed exceptions. Will need to extend/improve to match error strings. Fixes #4649