Patch demo API leaks PHP errors which leads to invalid json output

The patch demo instance has a full disk and as a result PHP is unable to write a session data under /var/lib/php/sessions and emit warnings:

<br />
<b>Warning</b>:  session_start(): open(/var/lib/php/sessions/sess_1tf6l5ou53nj305u0mu0ijuph8, O_RDWR) failed: No space left on device (28) in <b>/var/www/html/includes.php</b> on line <b>11</b><br />
<br />
<b>Warning</b>:  session_start(): Failed to read session data: files (path: /var/lib/php/sessions) in <b>/var/www/html/includes.php</b> on line <b>11</b><br />
[]

An example call https://patchdemo.wmflabs.org/api.php?action=findwikis&change=1023790

That causes the Gerrit plugin to error out as well:

SyntaxError: JSON.parse: unexpected character at line 1 column 1 of the JSON data