[Filesystem] Fix and complete the API

Implement the spec.
Fix and pass the TCT tests.
Fix and complete the example.

BUG=XWALK-1071
BUG=XWALK-1827
Seems I missed this nit comment, 4-space indent or move `error.name` can be end of last line because JS file is 100 chars limit, not 80 chars.