[dock] Use IOError in place of FileNotFoundError

FileNotFound is Python 3 only, therefore use its base class
to be compatible with both versions of Python.

beat/core/dock.py

@@ -543,7 +543,7 @@ class Host(object):
 
         try:
             process = sp.Popen(command, stdout=process_stdout, stderr=process_stderr)
-        except FileNotFoundError as e:
+        except IOError as e:
             if self.raise_on_errors:
                 raise