Summary:
Introduce by #95 'canonicalizePath' throws and exception when given
an invalid file in a call to 'sameFile'.

There are two cases when this can happen when using ghci:
  1) If there is an error at the interactive prompt, "<interactive>"
     file is searched for and not found.
  2) If there is an error in any loaded file and editing an inexistent/new
     file with 'e: foo'.

Both cases are now tested.

Test Plan: validate

Reviewers: austin, #ghc

Reviewed By: austin, #ghc

Subscribers: bgamari, thomie

Differential Revision: https://phabricator.haskell.org/D930

GHC Trac Issues: #10101