Conversion error caused by multiple CLI apps comes too late

[JamesHenry]

Background

We do not support converting projects which have multiple apps within their .angular-cli.json file.

Currently we check for this in updateAngularCLIJson() and throw:

if (angularCliJson.apps.length !== 1) {
  throw new Error('Can only convert projects with one app');
}

Issue

The problem is that updateAngularCLIJson() does not come until the fourth step in the conversion process:

export default function(schema: Schema): Rule {
  const options = { ...schema, name: toFileName(schema.name) };
  return chain([
    moveFiles(options),
    branchAndMerge(chain([mergeWith(apply(url('./files'), []))])),
    updatePackageJson(),
    updateAngularCLIJson(options), // <-- we don't throw until here
// ...etc...

This means that we move the user's files around and perform all kinds of modifications only to end up exiting early. This leaves the repo in a mess, which the user has to manually restore to where they started.

Proposed Solution

We introduce a new first step to the pipeline which contains relevant fundamental checks for whether or not the conversion should be possible.

[github-actions]

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