dumpJson.php will currently output error messages and progress reports to stdout, unless the dump itself is written to stdout, in which case progress and error reports are disabled.

dumpJson.php should instead:
a) report progress and errors to stderr per default
b) have a --logfile option to redirect progress reports and errors somewhere else

Version: unspecified
Severity: normal
See Also:
https://bugzilla.wikimedia.org/show_bug.cgi?id=54369