files: avoid surprises when linking files

In the unlikely (but possible) event that a directory is created in
place of a target file *after* `checkLinkTargets` but *before*
`linkGeneration`, currently the link is created *inside* the directory,
which is surprising.

To avoid this, use `ln -T` and bail if it fails.

Also bail if backing up the target fails.

Ideally, `checkLinkTargets` and `linkGeneration` should be more tightly
coupled.

modules/files.nix

@@ -177,7 +177,7 @@ in
             if [[ -e "$targetPath" && ! -L "$targetPath" && -n "$HOME_MANAGER_BACKUP_EXT" ]] ; then
               # The target exists, back it up
               backup="$targetPath.$HOME_MANAGER_BACKUP_EXT"
-              $DRY_RUN_CMD mv $VERBOSE_ARG "$targetPath" "$backup" || errorEcho "Moving '$targetPath' failed!"
+              $DRY_RUN_CMD mv $VERBOSE_ARG "$targetPath" "$backup" || exit 1
             fi
 
             if [[ -e "$targetPath" && ! -L "$targetPath" ]] && cmp -s "$sourcePath" "$targetPath" ; then
@@ -186,7 +186,7 @@ in
             else
               # Place that symlink, --force
               $DRY_RUN_CMD mkdir -p $VERBOSE_ARG "$(dirname "$targetPath")"
-              $DRY_RUN_CMD ln -nsf $VERBOSE_ARG "$sourcePath" "$targetPath"
+              $DRY_RUN_CMD ln -Tsf $VERBOSE_ARG "$sourcePath" "$targetPath" || exit 1
             fi
           done
         '';