Explicitly import PynagError

This is needed because there are still unresolved cross-imports
in pynag and explicitly importing PynagError fixes parts of the code
that Utils was import but at the same time those modules were doing:

'from pynag.Utils import PynagError'

This refactor, and a few other changes will make it possible for
us to get rid of those cross imports.

pynag/Utils/__init__.py

@@ -591,6 +591,7 @@ def run_command(command, raise_error_on_fail=False, shell=True, env=None):
 from pynag.Utils import checkresult
 from pynag.Utils import decorators
 from pynag import errors
+from pynag.errors import PynagError
 from pynag.Utils import git
 from pynag.Utils import metrics
 from pynag.Utils import nsca