Should stop execution if it can't find .inst/apps

[karangattu]

Fixes #234

The builds are failing though :(

[gadenbuie]

Sorry @karangattu, I just realized that the function signature of fix_snaps() already includes a repo_dir argument.

fix_snaps <- function(
  sha = git_sha(repo_dir),
  ...,
  ask_apps = FALSE,
  ask_branches = TRUE,
  ask_if_not_main = TRUE,
  repo_dir = "."
)

So it might be easier to change the default argument from repo_dir = "." to repo_dir = rprojroot::find_package_root_file().

You'd still keep the check that {repo_dir}/inst/apps exists and error if not, but the error message could point the user to change the repo_dir argument.

Also it'd be a good idea to check that all steps inside fix_snaps() take and respect the repo_dir argument (just by following the code, I mean).