Add error message to presubmit failure

Add error message to pre-upload script in the case where the remote branch does
not fit the standard '<remote>/<branch name>' format.

Previously the presubmit checks would error without printing the
relevant branch name.

BUG=None
TEST=repo upload .
Change-Id: I82545f3661b4f0403da71da73027b4b7e115d62a
Reviewed-on: https://chromium-review.googlesource.com/1179113
Commit-Ready: Josh Pratt <jopra@chromium.org>
Tested-by: Josh Pratt <jopra@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
diff --git a/pre-upload.py b/pre-upload.py
index 374b821..716c83b 100755
--- a/pre-upload.py
+++ b/pre-upload.py
@@ -1752,7 +1752,15 @@
           file=sys.stderr)
     remote = None
   else:
-    remote, _branch = remote_branch.split('/', 1)
+    branch_items = remote_branch.split('/', 1)
+    if len(branch_items) != 2:
+      PrintErrorForProject(
+          project_name,
+          HookFailure(
+              'Cannot get remote and branch name (%s)' % remote_branch))
+      os.chdir(pwd)
+      return True
+    remote, _branch = branch_items
 
   project = Project(name=project_name, dir=proj_dir, remote=remote)