Better error resilience against wrong user manager

Also improved get_class by using importlib instead of complicated
climbing through the whole module tree.

src/octoprint/server/__init__.py

@@ -210,7 +210,7 @@ class Server(object):
 		try:
 			clazz = octoprint.util.get_class(userManagerName)
 			userManager = clazz()
-		except AttributeError as e:
+		except:
 			self._logger.exception("Could not instantiate user manager {}, falling back to FilebasedUserManager!".format(userManagerName))
 			userManager = octoprint.users.FilebasedUserManager()
 		finally:

src/octoprint/util/__init__.py

@@ -174,8 +174,6 @@ def get_class(name):
 	"""
 	Retrieves the class object for a given fully qualified class name.
 
-	Taken from http://stackoverflow.com/a/452981/2028598.
-
 	Arguments:
 	    name (string): The fully qualified class name, including all modules separated by ``.``
 
@@ -183,15 +181,17 @@ def get_class(name):
 	    type: The class if it could be found.
 
 	Raises:
-	    AttributeError: The class could not be found.
+	    ImportError
 	"""
 
-	parts = name.split(".")
-	module = ".".join(parts[:-1])
-	m = __import__(module)
-	for comp in parts[1:]:
-		m = getattr(m, comp)
-	return m
+	import importlib
+
+	mod_name, cls_name = name.rsplit(".", 1)
+	m = importlib.import_module(mod_name)
+	try:
+		return getattr(m, cls_name)
+	except AttributeError:
+		raise ImportError("No module named " + name)
 
 
 def get_exception_string():

tests/util/test_misc.py

@@ -0,0 +1,31 @@
+# coding=utf-8
+from __future__ import absolute_import
+
+__license__ = 'GNU Affero General Public License http://www.gnu.org/licenses/agpl.html'
+__copyright__ = "Copyright (C) 2017 The OctoPrint Project - Released under terms of the AGPLv3 License"
+
+
+import unittest
+
+import octoprint.util
+
+class MiscTestCase(unittest.TestCase):
+
+	def test_get_class(self):
+		octoprint.util.get_class("octoprint.users.FilebasedUserManager")
+
+	def test_get_class_wrongmodule(self):
+		try:
+			octoprint.util.get_class("octoprint2.users.FilebasedUserManager")
+			self.fail("This should have thrown an ImportError")
+		except ImportError:
+			# success
+			pass
+
+	def test_get_class_wrongclass(self):
+		try:
+			octoprint.util.get_class("octoprint.users.FilebasedUserManagerBzzztWrong")
+			self.fail("This should have thrown an ImportError")
+		except ImportError:
+			# success
+			pass