DefaultWorkspaceManager: improve error handling in traverseFolder

[tomsontom]

fixes #2044

• check if directory exists eg in workspaces a directory could have been delete since the last start
• catch all errors while reading the directory content

[sailingKieler]

Thank you @tomsontom for taking the initiative about this! 🙏

I have an idea for an improvement, see below.

[sailingKieler]

What do you think about moving this diagnosis part into the catch block?, like

catch (e) {
    if(await this.fileSystemProvider.exists(folderPath) ) {
        console.error('Failure while reading directory content', e);
    } else {
        console.info(`Directory ${folderPath} does not exist`);
    }
}

[msujew]

On that note: Please don't use the sync file system methods in framework code - there is no guarantee that every file system is actually capable of supporting sync file system requests.

To be honest, I don't even see a reason to actually call exists at all, since the e error object will contain the information what kind of error occurred.