When looking up the IdP to authenticate, the try/catch block would
swallow the more specific exceptions that are generated and display
them all as a single 'not found' exception. This means that the
more specific "Metadata not found" exception page was never rendered,
or also the "Metadata expired" exception not displayed. We don't
need the try/catch here, instead can rely on the exceptions that this
call will generate by itself.

Incorrectly refactored in eb204573

Improve error handling

This reverts commit f1e05f8f.

The previous situation was confusing with various contradicting
(staments of) defaults, mixed with old style and new style
configuration.

Simplify by deciding that having backtraces is so
useful that it is the defaut, both old and new style configurations.
(It was already default on in the config template,
which seems 'canonical'.)

Although strictly 'less correct'. When manually converting
metadata, this is not such a useful note, and frequently gives
rise to trouble when the output is copied to somewhere static.

Previously, would show inappropriate precision, like "0.0598134 hours ago"
or "1490532.4129 hours from now" depending on the expiration date.