Clarify error message for missing module dependencies.

[ScottTodd]

For pipelines like https://github.com/iree-org/iree/blob/main/experimental/benchmarks/sdxl/sdxl_pipeline_bench_f16.mlir, the order that each .vmfb module is passed to tooling matters.

Running for example

iree-run-module \
  --module=sdxl_full_pipeline_fp16_rocm.vmfb \
  --module=sdxl_clip_vmfbs/model.rocm_gfx942.vmfb \
  --module=sdxl_unet_fp16_vmfbs/model.rocm_gfx942.vmfb \
  --module=sdxl_vae_vmfbs/model.rocm_gfx942.vmfb \
  ...

instead of

iree-run-module \
- --module=sdxl_full_pipeline_fp16_rocm.vmfb \
  --module=sdxl_clip_vmfbs/model.rocm_gfx942.vmfb \
  --module=sdxl_unet_fp16_vmfbs/model.rocm_gfx942.vmfb \
  --module=sdxl_vae_vmfbs/model.rocm_gfx942.vmfb \
+ --module=sdxl_full_pipeline_fp16_rocm.vmfb \
  ...

will hit this error message.

[geomin12]

beautiful! that helps clarify it