Improve Front-End Error Handling when DB is unreachable

[mrnoisytiger]

Steps to reproduce

1. Database (MySQL) is unreachable for whatever reason
2. Attempt to load Nextcloud

Expected behaviour

An error message mentioning that the database is unreachable at the current time

Actual behaviour

HTTP request hangs and does not complete. No front-end error is generated

Server configuration

Operating system: Ubuntu 14.04 LTS

Web server: Apache

Database: MySQL

PHP version: 7.0

Nextcloud version: 13.0.4

[nextcloud-bot]

GitMate.io thinks possibly related issues are #9441 (Improve token handling), #8928 (Improve error message when database is down/unavailable), #368 (Better error handling in occ), #10090 (Improved logging of smb connection errors), and #7513 (Improve handling of invalid federated (and regular) shares).

[ChristophWurst]

Dup of #8928