syntax: Display spans for open delimiters when a file ends prematurely

It's more helpful to list the span of each open delimiter seen so far
than to print out an error with the span of the last position in the file.

Closes #2354
syntax: Remove unnecessary @
auto merge of #9756 : catamorphism/rust/issue-2354, r=alexcrichton

r? anybody It's more helpful to list the span of each open delimiter seen so far
than to print out an error with the span of the last position in the file.

Closes #2354