Throw MultipleFailuresError when last retry failed

[nipafx]

Proposed commit message:

Better report assertion failures in retrying tests (#735 / #789)

IntelliJ (and maybe other tools as well) format test exception
messages in a way that highlights "expected" and "actual" values.
The way @RetryingTest repackages and such exceptions undermines that
special-casing and leads to less readable exception messages.

Change this by formatting the messages of `TestAbortedException`s, so
that the original exception's message shows up first (so it gets
presented well) and use `MultipleFailuresError` on the last failed
test to correctly report all previous errors.

Closes: #735
PR: #789

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