Rerun with missing input

[donkirkby]

Steps to reproduce:

1. Create an external dataset.
2. Use it as an input in a container run, and let the run finish.
3. Move the data file for the external dataset. It should then show up as a missing input on the container run page.
4. Rerun the container run.

Expected behaviour: display an error message, "Input some_input.fastq is missing. Cannot rerun."

Actual behaviour: unhelpful error message says something like, "Rerun failed."