fix: mds3 tk cnv legend will handle case when no cnv data is present …

[xzhou82]

…due to server error and not to crash

Description

closes #2863
fixes a problem Jian ran into in his branch (merge this first then rebase your on master)

test link, to break it, at line 1224 of mds3.init.js, change to bcfArgs.push('-s', l.join(',')+',xx') which causes an invalid sample name to be added to bcftools query command and triggers an error

the fix allows the error msg to be displayed in track body

Checklist

Check each task that has been performed or verified to be not applicable.

• Tests: added and/or passed unit and integration tests, or N/A
• Todos: commented or documented, or N/A
• Notable Changes: updated release.txt, prefixed a commit message with "fix:" or "feat:", added to an internal tracking document, or N/A