Truncate encoded instance message to 255 or fewer

In nova/compute/utils.py function exception_to_dict truncates unicode
message to 255. However, in non-English locales, instance message may
be longer than 255 after encoding the unicode message to byte message.
Need to truncate the encoded byte message to 255 or fewer, in order to
ensure db insert operation succeed.

Change-Id: I62fa2830b22e367eb9486d09d3c8818a18ebd20d
Closes-Bug: 1389102
Merge "Truncate encoded instance message to 255 or fewer"