[release] Improve error message for merge_to_release.py

The new message lets us know which directory is involved and what we can
do to fix it.

Bug: 
Change-Id: Icfcb92b35b1ef2644649789b1e8473cb6ae50336
Reviewed-on: https://chromium-review.googlesource.com/571702Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46659}

tools/release/common_includes.py

@@ -549,7 +549,8 @@ class Step(GitRecipesMixin):
   def InitialEnvironmentChecks(self, cwd):
     # Cancel if this is not a git checkout.
     if not os.path.exists(os.path.join(cwd, ".git")):  # pragma: no cover
-      self.Die("This is not a git checkout, this script won't work for you.")
+      self.Die("%s is not a git checkout. If you know what you're doing, try "
+               "deleting it and rerunning this script." % cwd)
 
     # Cancel if EDITOR is unset or not executable.
     if (self._options.requires_editor and (not os.environ.get("EDITOR") or