crypto/rsa: improve error message for keys too short for PSS

Fixes #23736

Change-Id: I850d91a512394c4292927d51c475064bfa4e3053
Reviewed-on: https://go-review.googlesource.com/92815Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/crypto/rsa/pss.go

@@ -36,7 +36,7 @@ func emsaPSSEncode(mHash []byte, emBits int, salt []byte, hash hash.Hash) ([]byt
 	// 3.  If emLen < hLen + sLen + 2, output "encoding error" and stop.
 
 	if emLen < hLen+sLen+2 {
-		return nil, errors.New("crypto/rsa: encoding error")
+		return nil, errors.New("crypto/rsa: key size too small for PSS signature")
 	}
 
 	em := make([]byte, emLen)