compute: using format_message() to convert exception to string

We probably need to use format_message when converting a
NovaException to a string.
As well we can remove the cast to unicode of the parameter msg
from the local method notify since this method will only get
message in a string format.

Change-Id: I6fddfb9e124b4096f0ebc3da81c3044b1b24cedf
Merge "compute: using format_message() to convert exception to string"