conversation: do not show error for non critical errors

If an error occurs during the execution of git_remote_fetch, there
is no need to signal any error to the client as it's not a critical
issue (no malformed conversation, mode is recognized, no unauthorized
method) and the sync will be retried later.

Change-Id: I2d875445e51aa6cd78eb2e7dbfe5efb8b2831860