chore(cli): improve error message when tree.json not found (#19887)

Related to #19234. This improves the log wording so it's more clear this is not a "hard stop" error, and switch the log level to "trace" so it won't be shown in as many ordinary logs as a false positive error.

----

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* [ ] Have you followed the guidelines in our [Contributing guide?](https://github.com/aws/aws-cdk/blob/master/CONTRIBUTING.md)

### Adding new Unconventional Dependencies:

* [ ] This PR adds new unconventional dependencies following the process described [here](https://github.com/aws/aws-cdk/blob/master/CONTRIBUTING.md/#adding-new-unconventional-dependencies)

### New Features

* [ ] Have you added the new feature to an [integration test](https://github.com/aws/aws-cdk/blob/master/INTEGRATION_TESTS.md)?
	* [ ] Did you use `yarn integ` to deploy the infrastructure and generate the snapshot (i.e. `yarn integ` without `--dry-run`)?

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packages/aws-cdk/lib/notices.ts

@@ -2,7 +2,7 @@ import * as https from 'https';
 import * as path from 'path';
 import * as fs from 'fs-extra';
 import * as semver from 'semver';
-import { debug, print } from './logging';
+import { debug, print, trace } from './logging';
 import { flatMap } from './util';
 import { cdkCacheDir } from './util/directories';
 import { versionNumber } from './version';
@@ -323,7 +323,7 @@ function loadTree(outdir: string) {
  try {
  return fs.readJSONSync(path.join(outdir, 'tree.json'));
  } catch (e) {
- debug(`Failed to get tree.json file: ${e}`);
+ trace(`Failed to get tree.json file: ${e}. Proceeding with empty tree.`);
  return {};
  }
 }