salt.modules.disk.percent() throws KeyError when partition doesn't exist

[arnisoph]

Thanks to @iigorr for reporting the issue!

Before:

# salt-call -l info disk.percent foo
[INFO    ] Executing command 'df -P' in directory '/root'
[ERROR   ] An un-handled exception was caught by salt's global exception handler:
KeyError: 'foo'
Traceback (most recent call last):
  File "/bin/salt-call", line 11, in <module>
    salt_call()
  File "/usr/lib/python2.7/site-packages/salt/scripts.py", line 331, in salt_call
    client.run()
  File "/usr/lib/python2.7/site-packages/salt/cli/call.py", line 51, in run
    caller.run()
  File "/usr/lib/python2.7/site-packages/salt/cli/caller.py", line 133, in run
    ret = self.call()
  File "/usr/lib/python2.7/site-packages/salt/cli/caller.py", line 196, in call
    ret['return'] = func(*args, **kwargs)
  File "/usr/lib/python2.7/site-packages/salt/modules/disk.py", line 203, in percent
    return ret[args]
KeyError: 'foo'
Traceback (most recent call last):
  File "/bin/salt-call", line 11, in <module>
    salt_call()
  File "/usr/lib/python2.7/site-packages/salt/scripts.py", line 331, in salt_call
    client.run()
  File "/usr/lib/python2.7/site-packages/salt/cli/call.py", line 51, in run
    caller.run()
  File "/usr/lib/python2.7/site-packages/salt/cli/caller.py", line 133, in run
    ret = self.call()
  File "/usr/lib/python2.7/site-packages/salt/cli/caller.py", line 196, in call
    ret['return'] = func(*args, **kwargs)
  File "/usr/lib/python2.7/site-packages/salt/modules/disk.py", line 203, in percent
    return ret[args]
KeyError: 'foo'

After:

# salt-call -l info disk.percent foo
[INFO    ] Executing command 'df -P' in directory '/root'
[ERROR   ] Problem parsing disk usage information: Partition 'foo' does not exist!
local:
    ----------

#27239 should be a working test for this case.