Improve error handling for anchoring strategies.
Avoid an error in the Range strategy when trying to create a TextPositionAnchor around a range with no content
Detect missing fields in TestPosition selectors.
Improve debug output
Update d-t-m to 271a373d (master branch)
TextPosition and TextQuote: Fix a bug on certain selections

Selections starting/ending at irrelevant DOM elements
(ie. DOM elements with no representation in the corpus)
could previous cause errors.

Now we are getting their positions correctly, too, using
the new APIs provided by d-t-m for this purpose.

Additionally, in the off-chance case when we somehow can't determine
positions, we avoid generating bogus TextPosition and TextQuote selectors.