WRITEME: example is quietly omitted when it has no snippet tags in the first excerpt

[Laren-AWS]

When an example has no snippet tags in the first excerpt, it is quietly omitted from the README.

For example, in this Java metadata, the first excerpt is only a description, which is valid. WRITEME detects the example has having no snippets and emits a subtle and rather cryptic warning and quietly omits the example from the output.

    Java:
      versions:
        - sdk_version: 2
          github: javav2/example_code/s3
          sdkguide:
          excerpts:
            - description: Generate a pre-signed URL for an object, then download it (GET request).
            - description: Imports.
              snippet_tags:
                - presigned.java2.generatepresignedgeturlandretrieve.import
            - description: Generate the URL.
              snippet_tags:
                - presigned.java2.generatepresignedgeturlandretrieve.createpresignedurl

Expected

WRITEME digs deeper into the excerpts to find a snippet tag so it can include the example.

[DavidSouther]

Also, Rachel found a variant where snippet tags from the Ailly template accidentally got higher priority, resulting in the "wrong" file path in the README. So it should choose the "best" by some heuristic.

[DavidSouther]

@scmacdon points out

aws-doc-sdk-examples/.doc_gen/metadata/cross_metadata.yaml

Line 49 in 420df9a

title_abbrev: Create a serverless application to manage photos

generates warnings for WRITEME. Whether these should find the "right" file, or WRITME should not care about these examples, is TBD.

[scmacdon]

I agree - Write Me should not care about these old X-SERs (they are still there) and move on to update Read Me based on YAML. S3 works now but SNS does not update Readme.