Example diff to illustrate this problem: "<mapframe>: Syntax error" is not very user-friendly. "JSON syntax error" or something is needed.
Or a wrapper error like "Unable to parse GeoJSON: Syntax error"
Change 292295 had a related patch set uploaded (by MaxSem):
Wrap JSON erros in a clarifyng message
Change 292295 merged by jenkins-bot:
Wrap JSON errors in a clarifyng message