Bug 1002040: Provide more informative page-move errors.

This tweaks the error handling in page move so that when the move
fails on a child (or grandchild, etc.) of the page being moved, the
generated error message will contain the ID, locale and slug of the
document the move failed on, as well as the type of exception it
raised and the original error message for diagnostic purposes.
Merge pull request #2357 from ubernostrum/page-move-error-1002040

Bug 1002040: Provide more informative page-move errors.