Bug #2057 - Messages from edit_xml operations do not show the xpath

Improved error messages for edit_xml promises:
- include more useful information in error messages, such as select_xpath and name of XML document being edited
- verify cfPS() parameters, such as cf_error or cf_verbose
- improve general appearance of messages, such as punctuation and whitespace

Correct SanityCheckTreeInsertions() so that there are not error messages on multiple passes, when inserting a tree into an initially empty file.