jsonfilter: improve error reporting

The CLI now describes expression errors more precisely, e.g.:

  Syntax error: Invalid escape sequence
  In expression @["foo\x"]
  Near here ----------^

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@41266 3c298f89-4303-0410-b956-a3cf2f4a3e73