tests: json_echo: Print errors to stderr

tests: json_echo: Print errors to stderr

Apart from the obvious, this fixes exit_dump() which tried to dump the
wrong variable ('out' instead of 'obj') and missed that json.dumps()
doesn't print but just returns a string. Make it call exit_err() to
share some code, which changes the prefix from 'FAIL' to 'Error' as a
side-effect.

While being at it, fix for a syntax warning with newer Python in
unrelated code.

Fixes: bb32d8db9a125 ("JSON: Add support for echo option")
Signed-off-by: Phil Sutter <phil@nwl.cc>