File name not always reported for input errors

[alexdewar]

There are a few input files we missed. The trick is to use .with_context(|| input_err_msg(filepath) as we've done elsewhere. I came across it because one of the input files had an error in and I had no idea which.

These are some of the files I think need changing:

• src/input/agent/region.rs
• src/input/commodity.rs
• src/input/commodity/demand.rs
• src/input/process.rs
• src/input/process/region.rs