document::element::operator[] overloads should not throw when given an invalid element

bsoncxx::document::element::operator[] is documented as non-throwing, however the expression bsoncxx::document::element{}["foo"] throws.

A fix for this issue was attempted in CXX-862, but the fix was incomplete.

Original description:

bsoncxx::document view blubb;

if( blubb["jolo"] )
=> false

if( blubb["jolo"][1] )
terminate called after throwing an instance of 'bsoncxx::v_noabi::exception'
what(): unset document::element
For this, can the exception say, that I'm out of bound of a array, or that no array is existing?
Most awesome would be if the exception would tell the non existing position.

This behavior is a bit misleading. I would assume for both the upper behavior in just returning a true or false.