Fix BeginTransaction#transaction error in rollback (#101)

When the rescued exception was caused by a protocol error, the
connection is in a corrupted state and executing the rollback
command might fail due to a subsequent protocol error - which is
irrelevant because the source of the corruption is described by
the original exception which must not be overlayed by a secondary one.

src/db/begin_transaction.cr

@@ -2,22 +2,29 @@ module DB
   module BeginTransaction
     # Creates a transaction from the current context.
     # If is expected that either `Transaction#commit` or `Transaction#rollback`
-    # are called explictly to release the context.
+    # are called explicitly to release the context.
     abstract def begin_transaction : Transaction
 
     # yields a transaction from the current context.
     # Query the database through `Transaction#connection` object.
     # If an exception is thrown within the block a rollback is performed.
-    # The exception thrown is blubbled unless it is a `DB::Rollback`.
+    # The exception thrown is bubbled unless it is a `DB::Rollback`.
     # From the yielded object `Transaction#commit` or `Transaction#rollback`
     # can be called explicitly.
     def transaction
       tx = begin_transaction
       begin
         yield tx
-      rescue e
+      rescue DB::Rollback
         tx.rollback unless tx.closed?
-        raise e unless e.is_a?(DB::Rollback)
+      rescue e
+        unless tx.closed?
+          # Ignore error in rollback.
+          # It would only be a secondary error to the original one, caused by
+          # corrupted connection state.
+          tx.rollback rescue nil
+        end
+        raise e
       else
         tx.commit unless tx.closed?
       end