Fix possible data loss upstream

[juanpicado]

Type: feature

Description:
Author: @Meeeeow
Follow up of #300

• Throw error when the error code is not ENOENT during reading package database file to prevent possible data loss
• Add warning about PM2 cluster mode to docs
• new: Add unit test for locking feature

Result

If for any reason the database is corrupted, the user will see that message. It won't be able to publish packages.

➜ npm publish                                 
npm ERR! publish Failed PUT 422
npm ERR! code E422
npm ERR! Verdaccio database is locked, please contact your administrator to checkout logs during verdaccio startup. : npm-test-hello-world

npm ERR! A complete log of this run can be found in:
npm ERR!     /Users/user/.npm/_logs/2017-08-25T18_24_48_546Z-debug.log

Resolves #??? ==> @Meeeeow could you fill this part?

[lock]

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.