sentence validation needs to explain the errors

[fcbr]

• and not just return t or nil.

Also: ideally it should provide the data in a structured way so that visual editors can use it to highlight the errors in the original file, not just display the error message in the screen. This is intentionally vague -- we need to define this better, of course.

[arademaker]

Related with

UniversalDependencies/tools#1 and
UniversalDependencies/tools#20

we need a better validation