Fixed a listing in README.org

* README.org: fixed expected output of first listing
Simplified and improved conditions in src/conditions.lisp

* src/conditions.lisp (header): updated copyright
  (location-error): made initargs required
  (xpath-creation-error): likewise; use
  `more-conditions:maybe-print-explanation' instead of
  `%maybe-add-explanation' in report
  (no-conversion-method-mixin): made initargs required
  (xml->-conversion-error): use
  `more-conditions:maybe-print-explanation' instead of
  `%maybe-add-explanation' in report
  (no-xml->-conversion-method): added default initargs
  (->xml-conversion-error): made initargs required; use
  `more-conditions:maybe-print-explanation' instead of
  `%maybe-add-explanation' in report
  (no-->xml-conversion-method): added default initargs
  (%maybe-add-explanation): removed; no longer required
* src/package.lisp (package xml.location): added used package
  more-conditions
* xml.location.asd (system xml.location): added system dependency on
  more-conditions
Added condition invalid-result-type in src/conditions.lisp

* src/conditions.lisp (result-condition): renamed
  result-set-error-mixin -> result-condition
  (invalid-result-type): new condition class; signaled when an XPath
  result has a wrong type
  (too-many-matches-in-result-set): changed superclass
  result-set-error-mixin -> result-condition
* src/location.lisp (header): updated copyright
  (evaluate! location): ensure that the result of the evaluation is a
  node set, signal `invalid-result-type' otherwise
* src/package.lisp (package xml.location): changed exported symbols
  result-set-error-mixin -> result-condition,
  location-error-result-set -> result-condition-result; added exported
  symbol invalid-result-type
Improved documentation strings in src/location.lisp

* src/location.lisp (setf location-namespaces :after t location):
  improved documentation string
  (setf location-compiled-path :after t location): likewise
Added TODO for namespaces in README.org

* README.org: added section Namespaces with TODO status