improve handling router script paths

The `server:run` command switches the working directory before
starting the built-in web server. Therefore, the path to a custom
router script had to be specified based on the document root path
and not based on the user's working directory.
backport more error information from 2.6 to 2.3

The commit on master was:

server:run command: provide more error information

The server:run command didn't provide many information when the executed
command exited unexpectedly. Now, the process' exit code is passed through
and an error message is displayed.
minor #11844 [FrameworkBundle] improve handling router script paths (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
----------

[FrameworkBundle] improve handling router script paths

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

The `server:run` command switches the working directory before starting the built-in web server. Therefore, the path to a custom router script had to be specified based on the document root path and not based on the user's working directory.

Another option is to update the documentation (as started in symfony/symfony-docs#4194). Though I think the current behaviour is a bug. The intended behaviour can be derived from the command's help message:

> ```
If you have custom docroot directory layout, you can specify your own
 router script using --router option:

>   ./app/console server:run --router=app/config/router.php
```

As you can see, the path is specified based on the current working directory.

Commits
-------

0a16cf2 improve handling router script paths
[FrameworkBundle] Remove invalid markup
Merge branch '2.3' into 2.4

* 2.3: (35 commits)
  [Form] Fix PHPDoc for builder setData methods The underlying data variable is typed as mixed whereas the methods paramers where typed as array.
  fixed CS
  [Intl] Improved bundle reader implementations
  [Console] guarded against invalid aliases
  switch before_script to before_install and script to install
  fixed typo
  [HttpFoundation] Request - URI - comment improvements
  [Security] Added more tests
  remove `service` parameter type from XSD
  [Intl] Added exception handler to command line scripts
  [Intl] Fixed a few bugs in TextBundleWriter
  [Intl] Updated icu.ini up to ICU 53
  [Intl] Removed non-working $fallback argument from ArrayAccessibleResourceBundle
  Use separated function to resolve command and related arguments
  [SwiftmailerBridge] Bump allowed versions of swiftmailer
  [FrameworkBundle] Remove invalid markup
  [Intl] Added "internal" tag to all classes under Symfony\Component\Intl\ResourceBundle
  Remove routes for removed WebProfiler actions
  [Security] Fix usage of unexistent method in DoctrineAclCache.
  backport more error information from 2.6 to 2.3
  ...

Conflicts:
	.travis.yml
	src/Symfony/Component/DependencyInjection/Loader/YamlFileLoader.php
	src/Symfony/Component/DependencyInjection/Tests/Loader/XmlFileLoaderTest.php
	src/Symfony/Component/HttpKernel/Kernel.php
	src/Symfony/Component/Process/PhpExecutableFinder.php