xl/bootup: Upon bootup handle errors loading bucket and event configs. (#3287)

In a situation when we have lots of buckets the bootup time
might have slowed down a bit but during this situation the
servers quickly going up and down would be an in-transit state.

Certain calls which do not use quorum like `readXLMetaStat`
might return an error saying `errDiskNotFound` this is returned
in place of expected `errFileNotFound` which leads to an issue
where server doesn't start.

To avoid this situation we need to ignore them as safe values
to be ignored, for the most part these are network related errors.

Fixes #3275

cmd/net-rpc-client.go

@@ -21,6 +21,7 @@ import (
 	"crypto/tls"
 	"crypto/x509"
 	"errors"
+	"fmt"
 	"io"
 	"net"
 	"net/http"
@@ -80,8 +81,13 @@ func (rpcClient *RPCClient) dialRPCClient() (*rpc.Client, error) {
 	if rpcClient.secureConn {
 		hostname, _, splitErr := net.SplitHostPort(rpcClient.node)
 		if splitErr != nil {
-			return nil, errors.New("Unable to parse RPC address <" + rpcClient.node + "> : " + splitErr.Error())
-
+			err = errors.New("Unable to parse RPC address <" + rpcClient.node + "> : " + splitErr.Error())
+			return nil, &net.OpError{
+				Op:   "dial-http",
+				Net:  rpcClient.node + " " + rpcClient.rpcPath,
+				Addr: nil,
+				Err:  err,
+			}
 		}
 		// ServerName in tls.Config needs to be specified to support SNI certificates
 		conn, err = tls.Dial("tcp", rpcClient.node, &tls.Config{ServerName: hostname, RootCAs: globalRootCAs})
@@ -95,7 +101,12 @@ func (rpcClient *RPCClient) dialRPCClient() (*rpc.Client, error) {
 		case x509.HostnameError:
 			errorIf(err, "Unable to establish RPC to %s", rpcClient.node)
 		}
-		return nil, err
+		return nil, &net.OpError{
+			Op:   "dial-http",
+			Net:  rpcClient.node + " " + rpcClient.rpcPath,
+			Addr: nil,
+			Err:  err,
+		}
 	}
 	io.WriteString(conn, "CONNECT "+rpcClient.rpcPath+" HTTP/1.0\n\n")
 
@@ -104,7 +115,12 @@ func (rpcClient *RPCClient) dialRPCClient() (*rpc.Client, error) {
 	if err == nil && resp.Status == "200 Connected to Go RPC" {
 		rpc := rpc.NewClient(conn)
 		if rpc == nil {
-			return nil, errors.New("No valid RPC Client created after dial")
+			return nil, &net.OpError{
+				Op:   "dial-http",
+				Net:  rpcClient.node + " " + rpcClient.rpcPath,
+				Addr: nil,
+				Err:  fmt.Errorf("Unable to initialize new rpcClient, %s", errUnexpected),
+			}
 		}
 		rpcClient.mu.Lock()
 		rpcClient.rpcPrivate = rpc