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.

首先跟踪 YML 源路径和所引用帧路径的解析方式，使用 issue 中的示例路径复现帧缺失的情况。验证缺失帧会产生明确的失败，并且处理会按要求继续。

由索引模型根据 Issue 内容生成。