Improve metrics_gather.sh parsing process

[rzumer]

• make it fault-tolerant, at least for optional metrics like VMAF
• display proper error messages when a metric cannot be parsed
• possibly review the format used to collect metrics

<TD-Linux> the problem is that all the metric parsing is really simple
<TD-Linux> it expects toe return to basically have one metric per line
<TD-Linux> but if the parsing breaks it could add things like extra lines or other junk instead which would mess it up
<TD-Linux> a more rigid metrics_gather.sh, or a better format for returning the metrics datat would help a lot here