A failing action prevents it's subsequent transition from affecting workflow state

[NightJar]

Setting up Advanced Workflow for a demonstrative run through in order to regression test, I've found that the 5.x line will hang up and seemingly offer limited actions than expected upon submitting for approval.

In reality what has happened is that my installation has failed to send an email, and the workflow has failed to progress.

This is different from 4.0.1 in that I received a toast notification in the CMS declaring an issue with Mailer.php, but then still presenting me with the expected next step action of approve or reject changes.

[NightJar]

This may be an underlying cause to #358

[NightJar]

Testing again recently, this issue only seems to manifest if the users have invalid email addresses.

e.g. default admin being admin rather than e.g. admin@localhost which appeared to function fine.

I presume there was still an issue sending an email since I have no mail server, but the workflow processed through as expected. This indicates a change in the Mailer interface being the root of the issue, where swiftmailer now expects well formed email addresses, and throws errors if they're not (where as the SilverStripe 3.x mailer was more tolerant of this).

[NightJar]

Also having recreated this now, it is the cause of #358 - an approval user can still 'manually' step through the workflow until such point as there is no next step, e.g. in the default Review and approve template the final email (notify initiator of approval/rejection) results in:

note no workflow options, although the popup is open

Trying to see if there was a user based workaround I ran into #366 - I could not delete the blocked workflow to free up editing on the page.

[NightJar]

I've confirmed this to be a direct result of attempting to send an email to an invalid email address (e.g. Member.Email being a username like admin instead of a valid email).
So I'll close this as a duplicate of #358 - as @phptek has already made this observation there