fix(init.js): show a custom error when the submodule operation fails

This generally occurs to people because they copy the executable in their project folder. Even
though it has an extension on Windows, git thinks it is already being used as a prefix. I think this
might relate to how git is installed w.r.t the new experimental symlink support. In any case, we
speculate the user have the executable in the wrong place and show a relevant error.

fix #57

modules/actions/init.js

@@ -126,17 +126,25 @@ const autoVendor = async (info) => {
   }
 
   if (info.vendorStrategy === 'submodule') {
-    await runGitMaybeHost(info, [
-      'submodule',
-      'add',
-      '--force',
-      '--name',
-      LIBDRAGON_SUBMODULE,
-      '--branch',
-      LIBDRAGON_BRANCH,
-      LIBDRAGON_GIT,
-      info.vendorDirectory,
-    ]);
+    try {
+      await runGitMaybeHost(info, [
+        'submodule',
+        'add',
+        '--force',
+        '--name',
+        LIBDRAGON_SUBMODULE,
+        '--branch',
+        LIBDRAGON_BRANCH,
+        LIBDRAGON_GIT,
+        info.vendorDirectory,
+      ]);
+    } catch (e) {
+      if (!(e instanceof CommandError)) throw e;
+      // We speculate this is caused by the user, so replace it with a more useful error message.
+      e.message = `Unable to create submodule. If you have copied the executable in your project folder or you have a file named ${info.vendorDirectory}, removing it might solve this issue. Original error:\n${e.message}`;
+      throw e;
+    }
+
     return info;
   }