Add our own DB exception abstraction

Right now our API exports the Doctrine/dbal exception. As we've seen
with the dbal 3 upgrade, the leakage of 3rdparty types is problematic as
a dependency update means lots of work in apps, due to the direct
dependency of what Nextcloud ships. This breaks this dependency so that
apps only need to depend on our public API. That API can then be vendor
(db lib) agnostic and we can work around future deprecations/removals in
dbal more easily.

Right now the type of exception thrown is transported as "reason". For
the more popular types of errors we can extend the new exception class
and allow apps to catch specific errors only. Right now they have to
catch-check-rethrow. This is not ideal, but better than the dependnecy
on dbal.

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>