media: limit size of stderr being logged

Whenever a rendering commands emit a ton of errors (spotted a case
generating 125MB), the call to trim() makes PHP copy the stderr which
might exhaust the remaining available memory.

The new constant MediaHandler::MAX_ERR_LOG_SIZE = 65535 is used to
substr() the error log before it get trimmed and logged.

bug: 57985
Change-Id: Ibd4cf0331529d323a68273163ac230b07a6c6108