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/signature-jwt.go

@@ -42,13 +42,7 @@ const (
 )
 
 // newJWT - returns new JWT object.
-func newJWT(expiry time.Duration) (*JWT, error) {
-	if serverConfig == nil {
-		return nil, errServerNotInitialized
-	}
-
-	// Save access, secret keys.
-	cred := serverConfig.GetCredential()
+func newJWT(expiry time.Duration, cred credential) (*JWT, error) {
 	if !isValidAccessKey(cred.AccessKeyID) {
 		return nil, errInvalidAccessKeyLength
 	}