File not found error

[richardssam]

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.