Feature: download a block debugging tool

[dirkmc]

Sometimes retrievals fail because of a malformed CAR file. In the retrieval UI the error appears as

graphsync response to peer 12D3KooWNTiHg8eQsTRx8XV7TiJbq3379EgwG6Mo3V3MdwAfThsx did not complete:
  response status code RequestFailedContentNotFound

This is because if Graphsync cannot read a block it reports the block as "not found".

When debugging why retrievals fail, it would be useful to have a link to download a block, so as to surface the underlying reason why the block cannnot be retrieved. At the moment the only way to see the reason is by looking in the logs for gs-queryexecutor errors. Some examples:

2023-03-20T16:22:24.794+0200    ERROR   gs-queryexecutor        queryexecutor/queryexecutor.go:225
      failed to load link=QmeipC66GLhbAAcy8HaAaxqAFvT47kP5TaMzoSSxGS3EVJ, nBlocksRead=0, err=1 error occurred:
        * reading data for block QmeipC66GLhbAAcy8HaAaxqAFvT47kP5TaMzoSSxGS3EVJ from reader for piece baga6ea4seaqhhp3py2aeuxs6vsrxicrmptphjkanwizolvojxan6hq2m573riby:
           expected 1 as the cid version number, got: 1266

2023-03-20T16:38:15.140+0200    ERROR   gs-queryexecutor        queryexecutor/queryexecutor.go:225
      failed to load link=QmRd7BKD3ubYEGck6UESEfL2PJkzLr2oZhGyAC2dz8e8FB, nBlocksRead=0, err=1 error occurred:
        * reading data for block QmRd7BKD3ubYEGck6UESEfL2PJkzLr2oZhGyAC2dz8e8FB from reader for piece baga6ea4seaqhhp3py2aeuxs6vsrxicrmptphjkanwizolvojxan6hq2m573riby:
          malformed car; header is bigger than util.MaxAllowedSectionSize