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.
Merge pull request #330 from shaunamarie/bug2057-xml-messages

Bug #2057 - Messages from edit_xml operations do not show the xpath
Fix for 1804

At creation time we were always assigning 0600 as mode to the new file.
This fix stats the source file first and assigns the same mode as the
original file.
Acceptance tests are updated too.