Allow failing build if there are missing runtime assets

[dwilches]

🚀 feature request

Relevant Package

This feature request is for @angular/cli

Description

If you have in the assets configuration in angular.json:

    "assets": [
        ...
        {
            "glob": "jquery.min.js",
            "input": "./node_modules/jquery/dist",
            "output": "/assets/jquery"
        },
        ...
    ]

And the glob doesn't match anything, no error is generated. This means the build can be broken because the assets folder doesn't contain all required runtime assets, but we won't realize until runtime.

Describe the solution you'd like

The feature request is to add an array field files that can be used instead of glob to tell exactly which files to copy: `"files":

    "assets": [
        ...
        {
            "files": ["jquery.min.js", "jquery.min.map"],
            "input": "./node_modules/jquery/dist",
            "output": "/assets/jquery"
        },
        ...
    ]

Describe alternatives you've considered

I considered listing the jquery.min.js file in the scripts section of angular.json, but this solution doesn't work perfectly for cases when it's not a "js" file I want in the assets folder. For example, mapbox comes with a folder with images, that I need copied to the assets folder.

An alternatively feature implementation I considered recommending was you could add an option failIfNoMatch to make the build fail if the glob doesn't match anything: "failIfNoMatch": true. But this solution is not as useful as the files one though, as the glob jquery.min.{js,map} would succeed even if it finds only one of the required files.

[petebacondarwin]

Transferring to the CLI project.

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