Added error handling for editor.

rtv/exceptions.py

@@ -8,5 +8,10 @@ class SubredditError(Exception):
     def __init__(self, name):
         self.name = name
 
+class ProgramError(Exception):
+    "Problem executing an external program"
+    def __init__(self, name):
+        self.name = name
+
 class EscapeInterrupt(Exception):
     "Signal that the ESC key has been pressed"