Improve error message content if on long lines

Improved display of error messages that
need to highlight an error on an exceptionally long line (like thousands
of opening braces). The error message generator now trims long lines,
and still, hopefully always, show an indicator where the error is on
that line.

Fixes: #26

Author: richkadel

Cargo.lock

@@ -1,6 +1,6 @@
 [package]
 name = "json5format"
-version = "0.2.2"
+version = "0.2.3"
 authors = [
   "Rich Kadel <richkadel@google.com>",
   "David Tamas-Parris <davidatp@google.com>",