improve an error message

old:

    i1 : value ///"///
    currentString:1:1:(0):[1]: error: ERROR in string beginning here: read failed: Bad file descriptor
    stdio:1:1:(3): error: read failed: Bad file descriptor

new:

    i1 : value ///"///
    currentString:1:1:(0):[1]: error: EOF in string beginning here
    stdio:1:1:(3): --backtrace: token read error--