Rename msg to fatalErrorMessage. (#34)

This is to free up `msg` as a variable name.

src/Panic.hs

@@ -12,7 +12,7 @@ import Data.Typeable (Typeable)
 import Control.Exception as X
 
 -- | Uncatchable exceptions thrown and never caught.
-data FatalError = FatalError { msg :: Text }
+data FatalError = FatalError { fatalErrorMessage :: Text }
   deriving (Show, Typeable)
 
 instance Exception FatalError