Opbeans chained exceptions

[jalvz]

Have some Opbeans app to generate errors with nested exceptions, ideally with several "sibling" causes.

ref elastic/apm#40

[kuisathaverat]

we just added a data generator from Opbeans UI here https://github.com/elastic/observability-test-environments/pull/90 probably the request that generates nested exceptions, it is a candidate to add also.

[beniwohli]

FYI, opbeans-python now has a chained exception (no siblings though, that's not a thing in Python). It's the AssertionError: oh no error.

scripts/compose.py start 7.4 --with-opbeans-python