lib.filesystem.pathType: Improve error for non-existent paths

Previously it would fail with

  error: attribute 'nonexistent' missing

         at nixpkgs/lib/filesystem.nix:29:10:

             28|     if dirOf path == path then "directory"
             29|     else (readDir (dirOf path)).${baseNameOf path};
               |          ^
             30|

lib/filesystem.nix

@@ -22,10 +22,14 @@ in
     Returns the type of a path: regular (for file), symlink, or directory.
   */
   pathType = path:
+    if ! pathExists path
+    # Fail irrecoverably to mimic the historic behavior of this function and
+    # the new builtins.readFileType
+    then abort "lib.filesystem.pathType: Path ${toString path} does not exist."
     # The filesystem root is the only path where `dirOf / == /` and
     # `baseNameOf /` is not valid. We can detect this and directly return
     # "directory", since we know the filesystem root can't be anything else.
-    if dirOf path == path
+    else if dirOf path == path
     then "directory"
     else (readDir (dirOf path)).${baseNameOf path};
 

lib/tests/filesystem.sh

@@ -51,6 +51,7 @@ checkPathType "$PWD/directory" '"directory"'
 checkPathType "$PWD/regular" '"regular"'
 checkPathType "$PWD/symlink" '"symlink"'
 checkPathType "$PWD/fifo" '"unknown"'
+checkPathType "$PWD/non-existent" "error: evaluation aborted with the following error message: 'lib.filesystem.pathType: Path $PWD/non-existent does not exist.'"
 
 checkPathIsDirectory() {
     local path=$1