EZP-22283: More descriptive error for invalid DB params

I am currently trying to perform an upgrade of an eZ Publish 4 project to eZ Publish 5. The eZ4 project is up to date with the current Legacy master branch from github. These are the steps I performed, following the installation instructions at Github:

```bash
$ git clone https://github.com/ezsystems/ezpublish-community.git cms
$ cp -R /path/to/old/ez cms/ezpublish_legacy
$ rm -rf cms/ezpublish/legacy/vendor
$ cd cms
$ php -dmemory_limit=-1 ../bin/composer.phar install --prefer-dist
```
This got me the following error:
```
[PDOException]
SQLSTATE[HY000] [2002] No such file or directory
```

After some heavy debugging, I became aware that the settings from app/config/ezpublish_setup.yml are being used, and thought it would be nice if the error message would hint me to this instead of giving me just a generic error message.

This PR adds an additional error message with a hint where to find and fix the database settings:

```
[RuntimeException]
Unable to create Legacy DB handler mysql:host=;database=none. Please check your database settings in ezpublish_*.yml.

[PDOException]
SQLSTATE[HY000] [2002] No such file or directory
```

Cheers
:octocat: Jérôme