SI-6728 Fixes crash in parser on incomplete for expression

The parser was trying to take the position of an empty expression
which would crash. Son on the empty expression case in TreeBuilder#
makeWhile it tries to do that and, if that failed, gets a position
that wraps both the condition and the body.

I also made a slight improvement to the UnsupportedOperationEx
messages in Position.
Merge pull request #2028 from JamesIry/2.10.x_SI-6728

SI-6728 Fixes crash in parser on incomplete for expression