Error message when starting under DCOS

[sanmiguel]

Immediately after installing the package under DCOS, attempts to run most commands fail with a stacktrace like:

$ dcos riak cluster list
Traceback (most recent call last):
  File "<string>", line 577, in <module>
  File "click/core.py", line 716, in __call__
  File "click/core.py", line 696, in main
  File "click/core.py", line 1060, in invoke
  File "click/core.py", line 1060, in invoke
  File "click/core.py", line 1060, in invoke
  File "click/core.py", line 889, in invoke
  File "click/core.py", line 534, in invoke
  File "click/decorators.py", line 64, in new_func
  File "click/core.py", line 534, in invoke
  File "riak_mesos/commands/cmd_cluster.py", line 176, in cluster_list
  File "riak_mesos/cli.py", line 399, in api_request
  File "riak_mesos/cli.py", line 412, in framework_request
riak_mesos.cli.CliError: 'Unable to to find framework URL'
cli returned -1

This is caused by the scheduler not yet being started.

We should print a kinder error, perhaps pointing to dcos riak framework wait-for-service as the way to wait for the scheduler to start.