fix(compiler): Raise appropriate exception when modules are missing during dependency graph construction

[peblair]

Fixes #1480. Additionally, this PR modifies the missing module error message to contain the correct source locations. For example, given the following files:
tst.gr:

import Number from "./numberr"
print(Number.min(2, 3))

numberr.gr:

import Number from "number"
import Foo from "foo" // <- does not exist
export let min = Number.min

The following error is reported:

 λ grain git:(philip/file-missing-import) > grain tst.gr
File "././numberr.gr", line 2, characters 16-21:
Error: Missing file for module foo

[phated]

Should we normalize the path? In the example "././numberr.gr" should probably remove one of the ./

[marcusroberts]

Nice work!

[phated]

Looks good to me! I rebased and tested it with my LSP fix and things look great in the problems tab.

I can't replicate the duplicate relativity in the example (././numberr.gr) because my file paths are always absolute. If Philip or anyone else can replicate, please open an issue explaining how you did it.