make ec2 errors not useless

The ec2 error log message on an unexpected exception doesn't
include the actual exception. From the best I can tell this is
because some time in the past the log message might also have
been returned to the user. However the current code builds the
user message completely separately.

We are tickling a bug on ec2 in the volumes stack that causes
"Unexpected KeyError raised", however without the stack trace it's
really hard to understand where this went wrong.

Change-Id: Ia0f9e6f957fa8788dc9e87631a10bc9631bb930b
Merge "make ec2 errors not useless"