Add error message for invalid concurrency value

This commit adds a detailed error message when a user passes in a
concurrency outside the expected range (which is >= 0). Previously
we'd raise an unhandled IndexError because of going outside the list
bounds deeper in the code, which wasn't super useful or informative for
users. This fixes that by detecting when we have an invalid input up
front and print that out before we run anything.

Related to: #202

stestr/commands/run.py

@@ -170,6 +170,11 @@ def take_action(self, parsed_args):
  color = args.color
  abbreviate = args.abbreviate
  suppress_attachments = args.suppress_attachments
+ if concurrency and concurrency < 0:
+ msg = ("The provided concurrency value: %s is not valid. An "
+ "integer >= 0 must be used.\n" % concurrency)
+ stdout.write(msg)
+ return 2
  verbose_level = self.app.options.verbose_level
  stdout = open(os.devnull, 'w') if verbose_level == 0 else sys.stdout
  result = run_command(
@@ -308,6 +313,11 @@ def run_command(config='.stestr.conf', repo_type='file',
  exit(1)
  repo = util.get_repo_initialise(repo_type, repo_url)
  combine_id = None
+ if concurrency and concurrency < 0:
+ msg = ("The provided concurrency value: %s is not valid. An integer "
+ ">= 0 must be used.\n" % concurrency)
+ stdout.write(msg)
+ return 2
  if combine:
  latest_id = repo.latest_id()
  combine_id = six.text_type(latest_id)