add the key piece of the json parser error output: the bad input

[baallan]

When users send malformed json, we get urps such as

first_line   : 0
last line    : 0
first column : 0
last column  : 5
str: a cryptic message about colons, commas or curlies.

This patch includes 'input' in the message, which tends to be truncated from the complete input very near the error. With this, the admin has a modest chance of figuring out what message went wrong.