Include the file name when reporting Dhall errors

[hdgarrood]

For example, if I modify packages.dhall so that there is a syntax error:

$ spago build
spago: 
Error: Invalid input

2:9:
  |
2 | Welcome to your new Dhall package-set!
  |         ^
unexpected 't'
expecting ':', '=', or whitespace

It would be helpful if the error message included the file name "packages.dhall". Likewise for errors involving spago.dhall.

[f-f]

@hdgarrood wow, great catch.

I'm not sure where the problem is - since the spago.dhall imports packages.dhall, the error happens on the Dhall side of things (it should be this line), but I'm pretty sure there's an option to get nicer error reporting with filenames (i.e. I think the fix could just be replacing inputExpr with inputFile on that line)

[f-f]

Fix up in #241