fixed check when computing root blocklength to return current blocklength value when hitting a group

Author: tmontgomery

main/java/uk/co/real_logic/sbe/xml/Message.java

@@ -365,7 +365,7 @@ private int computeMessageRootBlockLength()
  {
  if (field.groupFields() != null)
  {
- return field.computedOffset();
+ return blockLength;
  }
  else if (field.type() != null)
  {
@@ -387,7 +387,7 @@ private void validateBlockLength(final Node node, final long specifiedBlockLengt
  {
  if (0 != specifiedBlockLength && computedBlockLength > specifiedBlockLength)
  {
- handleError(node, "specified blockLength provides insufficient space");
+ handleError(node, "specified blockLength provides insufficient space " + computedBlockLength + " > " + specifiedBlockLength);
  }
  }
 }