ACTION-2057 - Write spectext for replacement of compute-exception

All,

This one is done. For reference, discussed in:


https://lists.w3.org/Archives/Public/public-xformsusers/2016Mar/0007.html

and:

    https://www.w3.org/2016/03/16-forms-minutes.html#item01

Diff (which doesn't work):


https://www.w3.org/community/xformsusers/wiki/index.php?title=XForms_2.0&diff=222&oldid=213

But the list of changes is here:


https://www.w3.org/community/xformsusers/wiki/index.php?title=XForms_2.0&action=history

The diff also includes removal of a few remnants of references to
exceptions.

Finally, I also replaced `xforms-xpath-version-exception` with
`xforms-xpath-version-error` in the XPath Expressions Module:


https://www.w3.org/community/xformsusers/wiki/index.php?title=XPath_Expressions_Module&diff=224&oldid=167

This means that `-exception` event are entirely gone from XForms.

-Erik