Catch ImportErrors and improve coverage metrics

ddt.py

@@ -13,7 +13,7 @@ from functools import wraps
 
 try:
     import yaml
-except Exception:
+except ImportError:  # pragma: no cover
     _have_yaml = False
 else:
     _have_yaml = True

test/test_example.py

@@ -4,7 +4,7 @@ from test.mycode import larger_than_two, has_three_elements, is_a_greeting
 
 try:
     import yaml
-except Exception:
+except ImportError:  # pragma: no cover
     have_yaml_support = False
 else:
     have_yaml_support = True