Provide convenient access to XML element name

[jclark]

Assuming

• we will allow E.x to provide access to the fields of the error detail record E.x on error value should access error detail record field #221, and
• we will allow E.x to access XML element attributes (as proposed in Convenient XML navigation #169 (comment))

we should provide an equally convenient way to access the error reason string and XML element name (this would also work for the XML processing instruction target). For the XML element name case, it should be an lvalue.

At least in the XML case, it is reasonable to think of the XML element name as being a special unnamed attribute, which suggests using E._ as the syntax.

[jclark]

Assuming we change errors to use branding (#413, #459), then this will apply only to XML elements and processing instructions.