Handle Errors returns from geturlpart() in json function

[jacobmealey]

This PR adds a trurl_warnf call in json() to have it handle bad return codes from geturlpart() where previously it was silently failing. this aligns the behavior of --get and --json. This silent error also meant trurl would attempt to print all params even if the call to geturlpart fails which is now fixed.

this pr sort of fixes #262. like I said above it aligns --get and --json so if there are control sequences in the input they will both return the same error unless you call it with --urlencode