build(docs-infra): fail gently if a changed file is unknown (#40479)

When using `docs-watch` we try to match the changed file
to a dgeni package in order to trigger a minimal set of processors.

Previously, if the changed file could not be matched we emitted an
ugly error. Now we provide a helpeful message.

PR Close #40479

aio/tools/transforms/authors-package/index.js

@@ -51,6 +51,10 @@ module.exports = {
   generateDocs: function(changedFile, options = {}) {
     const {Dgeni} = require('dgeni');
     const package = createPackage(changedFile);
+    if (package === undefined) {
+      console.log('The changed file was not matched to a dgeni package - skipping doc-gen');
+      return Promise.resolve();
+    }
     if (options.silent) {
       package.config(function(log) { log.level = 'error'; });
     }