Explicit message if unsufficient credentials

[odeckmyn]

Here we should have logged in Azure before invoking command:

dmake --azure release --push

Traceback (most recent call last):
 File "/Users/username/.pyenv/versions/3.7.5/bin/dmake", line 10, in <module>
  sys.exit(main())
 File "/Users/username/.pyenv/versions/3.7.5/lib/python3.7/site-packages/dmake/cmd.py", line 268, in main
  azure_manager.setup()
 File "/Users/username/.pyenv/versions/3.7.5/lib/python3.7/site-packages/dmake/azure.py", line 178, in setup
  if not self.get_ecr_repositories():
 File "/Users/username/.pyenv/versions/3.7.5/lib/python3.7/site-packages/dmake/azure.py", line 136, in get_ecr_repositories
  describe = self.az("acr list", capture=True)
 File "/Users/username/.pyenv/versions/3.7.5/lib/python3.7/site-packages/dmake/azure.py", line 47, in az
  return self.system("az {}".format(" ".join(args)), **kwargs)
 File "/Users/username/.pyenv/versions/3.7.5/lib/python3.7/site-packages/dmake/base_commands.py", line 353, in system
  verbose=self.verbose,
 File "/Users/username/.pyenv/versions/3.7.5/lib/python3.7/site-packages/dmake/common.py", line 92, in system
  raise OSError(ret)