Make utils.getPackageDir raise correct exception

assertRaises with isInstance test of correct exception, and note about why
it looks strange.

python/lsst/utils/__init__.py

@@ -22,6 +22,8 @@
 #
 from __future__ import absolute_import
 
+import lsst.pex.exceptions
+
 from .raDecStr import *
 from .demangle import *
 from .utils import *

tests/testGetPackageDir.py

@@ -32,7 +32,12 @@ def testBasics(self):
         utilsPath = getPackageDir("utils")
         self.assertTrue(os.path.isfile(os.path.join(utilsPath, "tests", "testGetPackageDir.py")))
 
-        self.assertRaises(Exception, getPackageDir, "nameOfNonexistendPackage2234q?#!")
+        # NOTE: one goal of this test is to confirm that the correct exception
+        # is raised even if we haven't explicitly imported pex.exceptions.
+        # Hence the odd structure of this assert block.
+        with self.assertRaises(Exception) as cm:
+            getPackageDir("nameOfNonexistendPackage2234q?#!")
+        self.assertIsInstance(cm.exception, lsst.pex.exceptions.NotFoundError)
 
     def testUnicodeBasics(self):
         utilsPath = getPackageDir(u"utils")