Error email improvements

[marcinapostoluk]

• Separating out error email so that it can be specified independently via UI. This is so that successful reports can be sent to target audience while errors to notebook maintainer
• If no error email is specified errors won't be sent to the regular email any more. The assumption here is that notebook maintainer will monitor it manually. This is a change of behavior as after an upgrade to the new version emails will no longer be sent on failure unless changes the config explicitly. If strong objections I can default it to use regular email although this disallows configuring a case where we do want regular emails sent but not the error emails.
• Storing regular and error emails together in each notebook result - in case one wants to re-run we need to have them both as the result of the re-run can be either a success or a failure

[jonbannister]

If no error email is specified errors won't be sent to the regular email any more. The assumption here is that notebook maintainer will monitor it manually.

I don't think this is a safe assumption. Plenty of internal users only notice that their scheduled report is failing when they get the email (many don't check the webapp at all).

[marcinapostoluk]

If no error email is specified errors won't be sent to the regular email any more. The assumption here is that notebook maintainer will monitor it manually.

I don't think this is a safe assumption. Plenty of internal users only notice that their scheduled report is failing when they get the email (many don't check the webapp at all).

Sure, no problem, changed to default to regular email now if error email not specified

[jonbannister]

• Please add to the CHANGELOG

[jonbannister]

LGTM