Enable Session.ExecuteBatchCAS() to return underlying scan errors

The patch enables Session.ExecuteBatchCAS() to return an error which may happen
during rows scanning.

Patch by Bohdan Siryk; reviewed by <> for CASSGO-47

Author: worryg0d

CHANGELOG.md

@@ -18,6 +18,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - Change Batch API to be consistent with Query() (CASSGO-7)
 
+- Enable Session.ExecuteBatchCAS() to return underlying scan errors (CASSGO-47)
+
 ### Fixed
 
 - Retry policy now takes into account query idempotency (CASSGO-27)

session.go

@@ -769,7 +769,7 @@ func (s *Session) ExecuteBatch(batch *Batch) error {
 // ExecuteBatchCAS executes a batch operation and returns true if successful and
 // an iterator (to scan additional rows if more than one conditional statement)
 // was sent.
-// Further scans on the interator must also remember to include
+// Further scans on the iterator must also remember to include
 // the applied boolean as the first argument to *Iter.Scan
 func (s *Session) ExecuteBatchCAS(batch *Batch, dest ...interface{}) (applied bool, iter *Iter, err error) {
 	iter = s.executeBatch(batch)
@@ -785,7 +785,7 @@ func (s *Session) ExecuteBatchCAS(batch *Batch, dest ...interface{}) (applied bo
 		iter.Scan(&applied)
 	}
 
-	return applied, iter, nil
+	return applied, iter, iter.err
 }
 
 // MapExecuteBatchCAS executes a batch operation much like ExecuteBatchCAS,