dev: improve error message for dest_does_not_exist

This error message was ambiguous for mkdir operations where a shortcut
target is specified for hl_mkdir. The message looks like it's referring
to the shortcut target but really it is referring to the parent
directory for the new directory that will be created.

src/backend/src/api/APIError.js

@@ -121,7 +121,13 @@ class APIError {
         },
         'dest_does_not_exist': {
             status: 422,
-            message: 'Destination was not found.',
+            message: ({ what_dest }) => {
+                if ( ! what_dest ) {
+                    return 'Destination was not found.';
+                }
+
+                return `Destination of ${quot(what_dest)} was not found.`;
+            },
         },
         'source_does_not_exist': {
             status: 404,

src/backend/src/filesystem/hl_operations/hl_mkdir.js

@@ -480,7 +480,7 @@ class HLMkdir extends HLFilesystemOperation {
             dir.get_selector_of_type(NodePathSelector);
 
         if ( ! maybe_path_selector ) {
-            throw APIError.create('dest_does_not_exist');
+            throw APIError.create('dest_does_not_exist', null, { what_dest: 'path from selector' });
         }
 
         const path = maybe_path_selector.value;
@@ -498,7 +498,12 @@ class HLMkdir extends HLFilesystemOperation {
 
     async _get_existing_top_parent ({ top_parent }) {
         if ( ! await top_parent.exists() ) {
-            throw APIError.create('dest_does_not_exist');
+            throw APIError.create('dest_does_not_exist', null, {
+                // This seems verbose, but is necessary information when creating
+                // shortcuts, otherwise the developer doesn't know if we're talking
+                // about the shortcut's target directory or this parent directory.
+                what_dest: 'parent directory of the new directory being created',
+            });
         }
 
         if ( ! top_parent.entry.is_dir ) {