Improve Import Error Message When Failure Originates from a Transitive Import

[bmarcaur]

From an internal slack discussion:

> Task :module-api:rawIr FAILED
Encountered error trying to parse file '/Volumes/git/repo/module-api/build/conjure/Service.yml'
Import not found for namespace: Namespace{name=othernamespace}

the actual import failure was while parsing a file imported by Service.yml, not in Service.yml itself (but it was particularly confusing because othernamespace was imported into Service.yml as well).

Here is a simple example of the issue:

# Service.yaml
types:
  conjure-imports:
    othernamespace: ./other-import.yml
    transitive: ./transitive-import.yml
  definitions:
    objects:
      OtherThing: othernamespace.thing
      TransitiveThing: transitive.thing

# transitive-import.yml
types:
  # intentionally empty for example, but should have:
  # othernamespace: ./other-import.yml
  conjure-imports: []
  definitions:
    objects:
      # references a namespace that is not declared, but is declared in the parent
      OtherThing: othernamespace.thing

The error message should more accurately reflect the enclosing file that failed to import.