[helix-rest] Surface validation reason in addVirtualTopologyGroup 400 response

[LZD-PratyushBhatt]

The addVirtualTopologyGroup IllegalArgumentException handler in ClusterAccessor returned only the echoed payload and command name, so the actual precondition failure (for example "Number of virtual groups cannot be greater than the number of zones.") was visible only in the helix-rest server log. Callers such as ACM received an opaque "Illegal input ... for command addVirtualTopologyGroup" with no cause.

Append ex.getMessage() to the badRequest body so the reason travels on the wire, and add a test asserting the reason is present in the 400 body.

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