[SPARK-26564] Fix wrong assertions and error messages for parameter checking

Additional fix to respect the existing error checking and revise the messages

Author: sekikn

sql/core/src/main/scala/org/apache/spark/sql/execution/exchange/BroadcastExchangeExec.scala

@@ -79,7 +79,7 @@ case class BroadcastExchangeExec(
           val (numRows, input) = child.executeCollectIterator()
           if (numRows >= 512000000) {
             throw new SparkException(
-              s"Cannot broadcast the table with more than 512 millions rows: $numRows rows")
+              s"Cannot broadcast the table with 512 million or more rows: $numRows rows")
           }
 
           val beforeBuild = System.nanoTime()

sql/core/src/main/scala/org/apache/spark/sql/execution/joins/HashedRelation.scala

@@ -413,7 +413,7 @@ private[execution] final class LongToUnsafeRowMap(val mm: TaskMemoryManager, cap
 
   private def init(): Unit = {
     if (mm != null) {
-      require(capacity <= 512000000, "Cannot broadcast more than 512 millions rows")
+      require(capacity < 512000000, "Cannot broadcast 512 million or more rows")
       var n = 1
       while (n < capacity) n *= 2
       ensureAcquireMemory(n * 2L * 8 + (1 << 20))