VM Service incorrectly wraps `compileExpression` error responses from external clients

[bkonyi]

When an external client providing a compileExpression service returns an error in response to a request, it should return an RPC error along the lines of {'error': { 'code': 113, 'message': 'Expression compilation error', 'details': '$cfeErrorOutput'}}. However, the VM Service (DDS does not have this problem) will return as the response to the expression evaluation request: {'error': { 'code': 113, 'message': 'Expression compilation error', 'details': '{'error': { 'code': 113, 'message': 'Expression compilation error', 'details': '$cfeErrorOutput'}}'}}.

The VM service should just directly return the expression compilation error from the external expression compilation service instead of wrapping its response.