Provide an error message when invalid transport is set

[rallytime]

Fixes a bug where if transport: foo was set in the minion config file, the minion would stracktrace upon startup:

root@rallytime:~# salt-minion -ldebug
[DEBUG   ] Reading configuration from /etc/salt/minion
[DEBUG   ] Including configuration from '/etc/salt/minion.d/_schedule.conf'
[DEBUG   ] Reading configuration from /etc/salt/minion.d/_schedule.conf
[DEBUG   ] Including configuration from '/etc/salt/minion.d/beacons.conf'
[DEBUG   ] Reading configuration from /etc/salt/minion.d/beacons.conf
[DEBUG   ] Using cached minion ID from /etc/salt/minion_id: rallytime
[DEBUG   ] Configuration file path: /etc/salt/minion
[WARNING ] Insecure logging configuration detected! Sensitive data may be logged.
[INFO    ] Setting up the Salt Minion "rallytime"
Process Process-1:
Traceback (most recent call last):
  File "/usr/lib/python2.7/multiprocessing/process.py", line 258, in _bootstrap
    self.run()
  File "/usr/lib/python2.7/multiprocessing/process.py", line 114, in run
    self._target(*self._args, **self._kwargs)
  File "/root/SaltStack/salt/salt/scripts.py", line 110, in minion_process
    minion.start()
  File "/root/SaltStack/salt/salt/cli/daemons.py", line 344, in start
    super(Minion, self).start()
  File "/root/SaltStack/salt/salt/utils/parsers.py", line 942, in start
    self.prepare()
  File "/root/SaltStack/salt/salt/cli/daemons.py", line 329, in prepare
    import salt.daemons.flo
  File "/root/SaltStack/salt/salt/daemons/flo/__init__.py", line 26, in <module>
    from . import core
  File "/root/SaltStack/salt/salt/daemons/flo/core.py", line 23, in <module>
    from raet import raeting
ImportError: No module named raet

Instead of stacktracing, we should provide a helpful error to the user and then exit.

root@rallytime:~# salt-minion -ldebug
[DEBUG   ] Reading configuration from /etc/salt/minion
[DEBUG   ] Including configuration from '/etc/salt/minion.d/_schedule.conf'
[DEBUG   ] Reading configuration from /etc/salt/minion.d/_schedule.conf
[DEBUG   ] Including configuration from '/etc/salt/minion.d/beacons.conf'
[DEBUG   ] Reading configuration from /etc/salt/minion.d/beacons.conf
[DEBUG   ] Using cached minion ID from /etc/salt/minion_id: rallytime
[DEBUG   ] Configuration file path: /etc/salt/minion
[WARNING ] Insecure logging configuration detected! Sensitive data may be logged.
[INFO    ] Setting up the Salt Minion "rallytime"
[ERROR   ] The transport 'foo' is not supported. Please use one of the following: tcp, raet, or zeromq. Exiting.
[INFO    ] The salt minion is shutting down..
[INFO    ] The Salt Minion is shut down

Fixes #36304