fix(base/conference): do not execute leave conference on web
fix(base/conference): do not execute leave conference on web

On web CONFERENCE_FAILED handlers are not setting the 'recoverable'
flag thus any middleware which rely on those should not execute on web.

gpolitis/jitsi-meet,bgrozev/jitsi-meet,gpolitis/jitsi-meet,gpolitis/jitsi-meet,jitsi/jitsi-meet,jitsi/jitsi-meet,jitsi/jitsi-meet,jitsi/jitsi-meet,jitsi/jitsi-meet,gpolitis/jitsi-meet,gpolitis/jitsi-meet,jitsi/jitsi-meet,gpolitis/jitsi-meet,gpolitis/jitsi-meet,gpolitis/jitsi-meet,bgrozev/jitsi-meet,bgrozev/jitsi-meet,jitsi/jitsi-meet,bgrozev/jitsi-meet,bgrozev/jitsi-meet,jitsi/jitsi-meet,bgrozev/jitsi-meet