mercurial/cffi/__init__.py

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.