Behavior when bounds are missing when selecting avg

[James-Crean]

Averages can not be taken across an axis that has no bounds. This currently throws as error, but the error message come out garbled. We need to fix the error message, and possibly apply a check for missing bounds.
If the bounds are missing, we should disable the ability to select an average at all since it wont work.

Steps to reproduce:

1. Select a variable such as clt from 'clt.nc'
2. When the dimensions are shown, avg the time axis and load the variable.
3. Plot clt. A garbled message will appear like the following:
   "('B', 'o', 'u', 'n', 'd', 's', ' ', 'n', 'o', 't', ' ', 'a', 'v', 'a', 'i', 'l', 'a', 'b', 'l', 'e', ' ', 't', 'o',...