Improve error messages and user feedback.

[stephank]

This PR is on top of #149.

There are a bunch of changes to error handling here:

• I've added BrokerError::SessionExpired, because I expect it to be a very common case, for which we can now give better feedback. The other classes of BrokerError::Input are all integration issues, and can use the generic template.
• I've added BrokerError::ProviderInput, which covers some of the previous cases for BrokerError::Provider and BrokerError::Input. This is specifically for errors related to the request input (thus 400), not provider communication (still 503). All BrokerError::Input is now related to the RP, while all BrokerError::Provider* is related to the provider. Error messages reflect this.
• Internal errors and rate limiting now no longer redirect back to the RP, and always display a message.
• Internal errors are now logged with a reference number, which is also visible on the error page. This number is also z-base-32, so it can also be read on the phone, for example.
• Error pages are now translated.

@onli Can you push a commit here with German translations for the new messages, if you can spare moment? :)

Example Dutch 'session expired' page:

Example Dutch 'internal error' page:

[coveralls]

Changes Unknown when pulling a3786c0 on featErrors into ** on master**.

[onli]

Hi @stephank. Very nice. I added the german translation and approved the PR.

[coveralls]

Changes Unknown when pulling 763b177 on featErrors into ** on master**.

[stephank]

Awesome, thanks! 😃