Tree, Combobox: check error return from dojo.data

[bill]

Tree (and maybe ComboBox) don't check for an error return from dojo.data, so if there's a problem with the data you never know. Should fix that.

[Douglas Hays]

(In [12149]) References #4860. Add onError for ComboBox?.

[bill]

(In [12705]) Final handling for reporting errors from dojo.data (which unfortunately come asynchronously). Fixes #4860.