Add transaction:Error type

Part of #267.

lang/lib/transaction.bal

@@ -14,13 +14,15 @@
 // specific language governing permissions and limitations
 // under the License.
 
+
+# Type of error returned by commit action.
+type Error distinct error;
+
 # Information about a transaction that does not change
 # after the transaction is started.
 type Info readonly & record {|
    # Unique identifier for the transaction branch
    byte[] xid;
-   // non-zero means this transaction was a retry of 
-   // a previous one
    # The number of previous attempts in a sequence of retries
    int retryNumber;
    # Information about the previous attempt in a sequence of retries.

lang/spec.html

@@ -8736,7 +8736,7 @@ <h5>Commit operation</h5>
 </p>
 <p>
 If the transaction manager decided to rollback, then the Commit operation
-returns the error value for the cause of the rollback. Otherwise, it returns
+returns the error value for the reason for the rollback. Otherwise, it returns
 nil.
 </p>
 
@@ -8845,8 +8845,9 @@ <h4>Commit action</h4>
 <p>
 Evaluating the <code>commit-action</code> performs the transaction manager
 Commit() operation; the result of this operation is the result of the
-<code>commit-action</code>. Note that the commit action does not alter the flow
-of control.
+<code>commit-action</code>. The type of the result is
+<code>transaction:Error?</code>. Note that the commit action does not alter the
+flow of control.
 </p>
 
 </section>