#3 Document validation/error message improvements

[Ifland]

Error messages in IE are a bit confusing with the
"anonymous" stack trace--the one that I got threw me
off until I looked at the same page in Firefox. After
looking closer, I did get the correct message telling
me that an <xforms:label> was missing (actually
mis-namespaced) from my <xforms:input> tag ("expected
label to not be null"--input.js, line 15.) The biggest
challenge was that the message wasn't obvious--maybe
something more like "The XForms document doesn't appear
to be valid: one or more of the input elements doesn't
contain a valid label element" would have helped
tremendously.

[John Kugelman]

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Will work on this later. Better error-handling and less
technical error messages are needed in many places.