Create an error mode session

[qstokkink]

As shown in #8194 (comment), some errors can occur before we have a GUI. In these cases, we should still launch some minimal "backup" GUI and notify the user of the error.

In particular for #8194 (comment), a corrupted private key, we should ask the user what they want to do with the corrupt material. There is no obvious solution to this: casually removing a private key is a very bad idea. However, we can give "human" advice instead of the raw traceback.

For other cases, unknown failures, the error should be reported "as usual".