lfs: improve the client message when the server signals an object error
Two things here. First, the previous message included a snippet of JSON, which
tends to be long (and in the case of lfs-test-server, has no error message).
Instead, give a concise message where possible, and leave the JSON to a debug
output. Second, the server can signal issues other than a missing individual
file. This change shows a corrupt file, but I'm debating letting the corrupt
file get downloaded, because 1) the error code doesn't really fit, and 2) having
it locally makes forensics easier. Maybe need a config knob for that.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}