Error handling when starting IRMA sessions in irma-client can be more flexible

[ivard]

When using the session: { start: ... } option in irma-client it is not possible to do nice error handling on custom errors.

Possible options are:

• Not using the start option and do a manual mapping; disadvantage is that the 'Try again' button will disappear on Error/Cancelled/Timeout.
• Not using the session option at all and start the session manually in a custom plugin (by saying session: false and handle the skipped transitions related to session manually in a custom plugin
• Returning the error that happens when rejecting the start() Promise.

A low-effort third option is missing, this can be improved.