Fix error message for ODK-only question types

Use the question type name rather than "Image capture".

http://manage.dimagi.com/default.asp?139419
Merge pull request #259 from dimagi/139419-odk-error

Fix error message for ODK-only question types
Show unified diff on assertXmlEqual fail

Much easier to read than full dumps of actual and expected XML.
Improve loading of malformed XForms
Merge pull request #263 from dimagi/142079-default-label-itext

Improve loading of malformed XForms
Merge branch 'master' into help-text