web: Add more data for jsonrpc responses. (#3296)

This change adds more richer error response
for JSON-RPC by interpreting object layer
errors to corresponding meaningful errors
for the web browser.

```go
&json2.Error{
   Message: "Bucket Name Invalid, Only lowercase letters, full stops, and numbers are allowed.",
}
```

Additionally this patch also allows PresignedGetObject()
to take expiry parameter to have variable expiry.

cmd/auth-rpc-client.go

@@ -65,7 +65,7 @@ type RPCLoginReply struct {
 
 // Validates if incoming token is valid.
 func isRPCTokenValid(tokenStr string) bool {
-	jwt, err := newJWT(defaultInterNodeJWTExpiry)
+	jwt, err := newJWT(defaultInterNodeJWTExpiry, serverConfig.GetCredential())
 	if err != nil {
 		errorIf(err, "Unable to initialize JWT")
 		return false