Goal: Fail gracefully if Arb library is not installed.

The current changes might be a path towards this goal, but more work is needed. Probably its good to first decide about a general approach towards these (semi-)optional imports.

CC: @mkoeppe @simon-king-jena

Component: refactoring

Keywords: sd111

Author: Tobias Diez

Branch/Commit: public/refactoring/arb @ 1522314

Issue created by migration from https://trac.sagemath.org/ticket/30716