Check XML files for well-formedness

[teobugslayer]

Check XML files for well-formedness. No validation is performed. The check should happen at least on build and livesync.

Whenever an invalid XML file is found, warn the user giving the exact location of the issue. Do not break the current operation.

Implementation detail: it happens actually on prepare.

[vchimev]

Important note - in {N} CLI 1.5.1 the xml files check for well-formedness will only warn the user for syntax errors, it will not abort any CLI operations.