Add a general dynamic error for "problem with XPath expression"

[ndw]

We have err:XS0107 for static errors in XPath expressions, but nothing equivalent for dynamic errors. We often have specific errors for specific problems, but we need a catch-all. (Reporting err:XS0107 is just...wrong)

[xml-project]

One use case would be (variants) of:

 <p:add-attribute attribute-name="att">
        <p:with-input><doc /></p:with-input>
        <p:with-option name="attribute-value" select="fn:upper-case(2)"></p:with-option>
</p:add-attribute>

This has to raise an error because the XPath expression in select has an error for the integer parameter.
But currently there is no error code for this. So my implementation has to use the generic XD0030. I would prefer an other error code being directly related to the XPath expression.

[ndw]

In the past few days, Achim and I have both stumbled over how to report errors in XPath expressions that aren't detected until runtime. For some errors of this form, we have specific error codes, but there's no general catch-all for "it didn't work." My implementation has been using err:XS0107, the static version of "invalid expression", but that's really inappropriate for a dynamic error.

I'm not sure where to add this to the specification, but I don't expect any pushback on the requirement, so I've proactively created a dynamic error for it:

err:XD0083: It is a dynamic error if an XPath expression cannot be evaluated, e.g. because the expression is not syntactically valid, contains reference to unknown variables, or makes use of undeclared functions.