ssh: return specific error for invalid signature algorithm

Previously, this would return the default error "no auth passed yet".

Not only is the new error more specific, it makes it easier to verify
the control flow of server authentication code.

Change-Id: I6c8de4e3f91da74274acbe9d87ec4f6158b4a94f
Reviewed-on: https://go-review.googlesource.com/c/142897
Run-TryBot: Han-Wen Nienhuys <hanwen@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

ssh/server.go

@@ -484,6 +484,7 @@ userAuthLoop:
 				// sig.Format.  This is usually the same, but
 				// for certs, the names differ.
 				if !isAcceptableAlgo(sig.Format) {
+					authErr = fmt.Errorf("ssh: algorithm %q not accepted", sig.Format)
 					break
 				}
 				signedData := buildDataSignedForAuth(sessionID, userAuthReq, algoBytes, pubKeyData)