[mythtv-cmyth ]Remove 'failed to connect to mythtv backend' notification

Currently if the addon is unable to connect to a previously working backend
(because backend is suspended or unreachable via network) it displays a pop up
every 10 or so seconds, which appears in all menus and over video/visualizations
and plays a notification sound.
[cmyth] Improve separator checking in cmyth_rcv_string

This improves the checking for string separators []:[].
In case a string ends with a part of a separator, the
code did not find the correct separator.

It was reported by a user that a recording "Title ["
breaks the loading of recordings because it does not
recognize the string's end. The protocol showed
"...[]:[]Title[[]:[]Subtitle...", and cmyth_rcv_string
returned "Title[[]:[]Subtitle".