Don't swallow exceptions when scanning the document

According to @csillag, there are no good (i.e. non-exceptional) reasons
why `this._scan()` should throw an error. As such, should not be
catching and swallowing the error, as this will make it much less likely
that we receive bug reports about brokenness in dom-text-mapper.

Inspired by conversation in #1596.
Merge pull request #1597 from hypothesis/dont-swallow-exceptions

Don't swallow exceptions when scanning the document