Handle tests which do not complete

If the return value hasn't been added to the dictionary, it means that the user's test didn't reach the end. This means that it raised an exception or was otherwise stopped early by pytest (skipped, failed, etc). In any case, we should not proceed with figure testing and use the result that pytest has already determined.

Fixes #172

pytest_mpl/plugin.py

@@ -622,6 +622,9 @@ def pytest_runtest_call(self, item):  # noqa
             wrap_figure_interceptor(self, item)
             yield
             test_name = generate_test_name(item)
+            if test_name not in self.return_value:
+                # Test function did not complete successfully
+                return
             fig = self.return_value[test_name]
 
             if remove_text:
@@ -749,5 +752,8 @@ def pytest_runtest_call(self, item):
         yield
         if get_compare(item) is not None:
             test_name = generate_test_name(item)
+            if test_name not in self.return_value:
+                # Test function did not complete successfully
+                return
             fig = self.return_value[test_name]
             close_mpl_figure(fig)