jar: Deal with non-sane manifests without erroring

[crozzy]

This is an error seen in the wild where an META-INF/MANIFEST.MF
file does not contain the Manifest-Version. Currently this will
cause an indexing failure (and a 500 response).

Signed-off-by: crozzy joseph.crosland@gmail.com

[crozzy]

These errors are mildly common in production:

{"level":"error","component":"internal/indexer/controller/Controller.Index","manifest":"sha256:79231af8f0170a588cb2e1ee3c9494d23e297d487316631542f4eb7f68bb41a2","state":"ScanLayers","error":"failed to scan all layer contents: invalid manifest version: \"\"","time":"2021-11-30T15:35:03Z","message":"error during scan"}

[crozzy]

Tooling around with the ways to create JARs I haven't able to reproduce or find a wild manifest without Manifest-Version yet

[crozzy]

Another error that would potentially be addressed with this

{"level":"error","component":"internal/indexer/controller/Controller.Index","manifest":"sha256:f7c39d2f065a867b1975eb32748980073a9bb3c618f356e7dabd5ee8e43f0b57","state":"ScanLayers","error":"failed to scan all layer contents: no headers found","time":"2021-11-29T17:52:09Z","message":"error during scan"}

[hdonnay]

LGTM

I'd change the key in that one log line, but that's not a blocker.