File not found error

If the yml source file exists, but the frames that the file points to do not, the error is rather obscure.

If the file is missing, the code currently ends up thinking that the image path is:
"/test/enctests/sources/enc_sources/chimera_fountains_srgb/chimera_fountains_srgb.%05d.png.yml%02d"
instead of:
"/test/enctests/sources/enc_sources/chimera_fountains_srgb/chimera_fountains_srgb.%05d.png"

We should be able to fail gracefully and continue.

首先追蹤 YML 來源路徑和所參照影格路徑的解析方式，使用 issue 中的範例路徑重現影格遺失的情況。驗證遺失的影格會產生明確的失敗，且處理會依要求繼續。

由索引模型根據 Issue 內容生成。