Truncate really long backtraces

[marshall007]

For example, when you're inserting a large number of docs with table.insert(docs, { bad_optarg: true }) the backtrace will include pretty-printed JSON of all the documents. This is especially problematic in cases where the error occurs early in the query. It would be much more useful if the object literals were omitted:

RqlCompileError: Unrecognized optional argument `bad_optarg` in:
r.db('test').table('test').insert([ ... ], {bad_optarg: true})
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^