gclient_eval: Raise an error on implicitly concatenated strings.

Implicitly concatenated strings are not supported when setting new
revisions, so raise an error when attempting to do se.

Bug: 1060772
Change-Id: I74d39b9eb8dcc420e9b7787c90bee642beb783cc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2099407
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

roll_dep.py

@@ -265,6 +265,8 @@ def main():
   except Error as e:
     sys.stderr.write('error: %s\n' % e)
     return 2 if isinstance(e, AlreadyRolledError) else 1
+  except subprocess.CalledProcessError:
+    return 1
 
   print('')
   if not reviewers: