Invalid strings for "Jump to", "Play Until", and "Continue at" should report error and/or displayed Red

If a jump has an invalid string for any marker ("Jump to", "Play Until", or "Continue at"), then I suggest that the jump text and the undefined marker string(s) be displayed red when viewed in musescore (like notes with pitches out of range are) so user can easily spot them as erroneous.

Alternatively, or in addition, present the user with some message - maybe as a *warning* when trying to save locally and maybe as an *error* when trying to upload - that says "Marker $name undefined in Jump at meas $num".

This feature request is spun off from the bug: https://musescore.org/en/node/66946 "Null/Empty/Invalid Jumps should either not interfere with playback".