kate: some minor clean-ups

kate: some minor clean-ups

Print flow return as string in log message; if we check the return
value of gst_buffer_new_and_alloc() we should use the _try() function
that might actually return NULL. Post error message when returning
GST_FLOW_ERROR. Use portable GLib macros to print 64-bit integers.
Don't use 0LL, that's also not portable (and unneeded here).