Result Plugins: handle exceptions when rendering the results

Previously, a crash in one result plugin could abort the execution
of other plugins and consequently the last parts of the avocado
command line application execution.

Let's handle exceptions that could be raised there, as it's already
being done with Job Pre/Post plugins.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

avocado/core/dispatcher.py

@@ -94,3 +94,17 @@ class ResultDispatcher(Dispatcher):
 
     def __init__(self):
         super(ResultDispatcher, self).__init__('avocado.plugins.result')
+
+    def map_method(self, method_name, result, job):
+        for ext in self.extensions:
+            try:
+                if hasattr(ext.obj, method_name):
+                    method = getattr(ext.obj, method_name)
+                    method(result, job)
+            except SystemExit:
+                raise
+            except KeyboardInterrupt:
+                raise
+            except:
+                job.log.error('Error running method "%s" of plugin "%s": %s',
+                              method_name, ext.name, sys.exc_info()[1])