Issue title: `assetic:watch` cannot dump asset which are registered by `assetic.asset` tag
 Issue body: `assetic:watch` raises an error `[error] There is no "{asset_alias}" asset.`, when some formulae contains assets which are registered by `asseitc.asset` tag.

I think that reloading assets of AssetManager might create some formulae which contains unset asset references.
https://github.com/symfony/assetic-bundle/blob/master/Command/WatchCommand.php#L73
 Comments: 
 Comment 0: I'm curious to see a use case for the usage of ``assetic.asset`` tag. this feature was introduced before I became a maintainer, and I don't see in which case it should be used. Can you describe your use case ?
 Comment 1: I'm using `assetic.asset` tag to use dynamic js sources as asset.
(converting multiple html files to one js file.)

And wanted to apply asset filter to it.
 Comment 2: @stof I've created PR #412 to fix this for now.
{'source': 'GithubIssues', 'repo_name_with_owner': 'symfony/assetic-bundle', 'issue_id': 129482612, 'repo_stars': 236.0}