make git a bit less cryptic on fetch errors

make git a bit less cryptic on fetch errors

The remote server might not want to tell why it doesn't like us for
security reasons, but let's make the client report such error in a bit
less confusing way.  The remote failure remains a mystery, but the local
message might be a bit less so.

[jc: with a gentle wording updates from Andy Parkins]

Signed-off-by: Nicolas Pitre <redacted>
Signed-off-by: Junio C Hamano <redacted>