If the `releases` directory exists, but still only a subset of the
necessary previous release binaries are available, the test fails by
throwing an exception (sometimes leading to follow-up exceptions like
"AssertionError: [node 0] Error: no RPC connection") and printing out
a stack trace, which can be confusing and at a first glance suggests
that the node crashed or some alike.
Improve this by checking and printing out *all* of the missing release
binaries and failing with an explicit error in this case. Also add an
info on how to download previous releases binaries.
Noticed while testing #30328.
Can be tested by e.g.
$ ./test/get_previous_releases.py -b
$ rm -rf ./releases/v28.0/
$ ./build/test/functional/wallet_migration.py
1ea7e45a1f