[core] Add debugging capabilities

* If an exception is thrown, catch it and show a (somewhat) nice error
  message in the i3bar instead of the normal content
* Add a flag "-d" for debugging into a debug log. Currently, this only
  logs commandline calls as they occur and their return values, as well
  as exceptions.

fixes #58

bumblebee/modules/error.py

@@ -0,0 +1,26 @@
+# pylint: disable=C0111,R0903
+
+"""Draws an error widget.
+"""
+
+import bumblebee.input
+import bumblebee.output
+import bumblebee.engine
+
+class Module(bumblebee.engine.Module):
+    def __init__(self, engine, config):
+        super(Module, self).__init__(engine, config,
+            bumblebee.output.Widget(full_text=self.text)
+        )
+        self._text = ""
+
+    def set(self, text):
+        self._text = text
+
+    def text(self, widget):
+        return self._text
+
+    def state(self, widget):
+        return ["critical"]
+
+# vim: tabstop=8 expandtab shiftwidth=4 softtabstop=4