Truncate encoded instance message to 255

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, in order to ensure
db insert operation succeed.

Change-Id: I70cd2d6e4590ea95510c1ea3d51287f0dea14956
Closes-Bug: 1389102
Merge "Truncate encoded instance message to 255"