bug #14129 [FrameworkBundle] Fixed server:start --router relative path issue #14124 (abulford)
bug #14129 [FrameworkBundle] Fixed server:start --router relative path issue #14124 (abulford)

This PR was squashed before being merged into the 2.6 branch (closes #14129).

Discussion
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[FrameworkBundle] Fixed server:start --router relative path issue #14124

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #14124
| License       | MIT
| Doc PR        | -

This ensures that a relative path can be used to specify the router script, and that if the router script cannot be found an error is reported and the command fails.

The file path for the router script is now determined and checked prior to the child process being forked, which seems cleaner.  The same change could be made in ServerRunCommand.php, or possibly in the ServerCommand base class; to keep the scope small, though, this wasn't done as part of this bug fix.

Commits
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0c1b9ba [FrameworkBundle] Fixed server:start --router relative path issue #14124

symfony/framework-bundle