Do not abort the whole install when importmap:require fails

[Amoifr]

Fixes #1015.

When PackageJsonSynchronizer synchronizes AssetMapper packages, it runs importmap:require, which reaches out to the network (the jsDelivr API). If that call fails (a rate limit, or just bad connectivity, as @nicolas-grekas hit during a workshop), the ScriptExecutionException propagates and aborts the whole composer require, leaving the app half configured with no obvious way for a newcomer to recover.

This makes that step fail safe: a failing importmap:require no longer aborts the install. The entries that could not be added are collected and reported at the end, with a hint to run composer install again once the network is back (the synchronization re-runs on every install and retries the missing entries).

Before:

Executing script importmap:require [KO]
 [KO]
Script importmap:require returned with error code 1
[the whole "composer require webapp" aborts]

After the recipes are still applied, and you get:

Could not add the following packages to your importmap: @hotwired/stimulus.
This is often caused by a temporary network issue. Run "composer install" again once your connection is back to add them.

This is the smaller, incremental fix. @nicolas-grekas, you mentioned you'd rather have PackageJsonSynchronizer not touch the network at all and defer it to a later stage (e.g. cache:clear). That is a larger change, happy to look into it separately if you'd prefer that direction instead.

Thanks!