Create the block folder at boot

[superboum]

The bug

Currently, Fs.writeFile(fullPath, 'PLACEHOLDER\n', w()); always fails silently.
It fails silently because the error is not checked on Fs.writeFile.
And it fails because the block folder is not created before we try to write the file.

Behaviour on the /checkup script

As a consequence, the first time you launch the server, the placeholder.txt file is not created.
When you run the checkup script, it fails to find the file but creates the missing folder as, as soon you ask to store a proper block, because the called logic has the Fse.mkdirp logic, and thus the folder is created. So, on the next restart of Cryptpad, the placeholder.txt file can be created, and magically, the checkup script succeed.

My patch

I create the block folder and check for error before trying to create placeholder.txt, then I also check for error during the placeholder.txt file creation.