bundle2: transmit exception during part generation If an exception is raised during a bundle2 part payload generation it is now recorded in the bundle. If such exception occurs, we capture it, transmit an abort exception through the bundle, cleanly close the current part payload and raise it again. This allow to generate valid bundle even in case of exception so that the consumer does not wait forever for a dead producer. This also allow to raise the exception during unbundling at the exact point it happened during bundling make debugging easier.

bundle2: add a interrupt mechanism It is now possible to emit a single part in the middle of a payload production. This part will be processed with limitation (only access to a `ui` object). The goal is to let the server raise exception and output while a part is being processed. The source motivation is to transmit exception that occurs while generating a part. This change is was the motivation to bump the bundle2 format from HG2X to HG2Y. Somehow, the format bump made it into 3.2 without it. So this change go on stable. It is low risk as bundle2 is still disabled by default.

transactions: change backupfiles format to use newlines Previously the journal.backupfiles file was delimited by \0. Now we delimit it using \n (same as the journal file). This allows us to change the number of values in each line more easily, rather than relying on the count of \0's.