Better enduser facing error handling

[mrvanes]

Currently satosa is completely transparent to the end-user, which is a logical result of it being a protocol proxy. If all goes well, the user should not know she visited satosa at all.

If anything goes wrong however, satosa lacks a generic templating framework to inform the user and developers start to invent their own wheels:

https://github.com/inacademia-development/svs/blob/a0c00b2280aa885598eb33ec1493e561e28d10d8/src/svs/user_consent.py#L49

https://github.com/SURFscz/SATOSA-micro_services/blob/master/src/satosa/micro_services/custom_alias.py

Wouldn't it be nice to have a generic templating framework accessible to satosa and all microservices?