util: throw toJSON errors when formatting %j

Documentation says:

• %j - JSON. Replaced with the string '[Circular]' if the argument contains circular references.

Correct me if I'm wrong, but I believe because of that this is a semver-patch.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Affected core subsystem(s)

util