Improve logging of AssertionErrors

Currently, when we have an assertion error raised from cmdlib, it looks like this:

  [cluster] root@node4:~# gnt-instance grow-disk instance1 0 1G
  Failure: command execution error:

This is very very confusing. This patch adds a bit of traceback
formatting to improve this as follows:

  [cluster] root@node4:~# gnt-instance grow-disk instance1 0 1G
  Failure: command execution error:
  Internal assertion error: please report this as a bug.
  Error message: ''; location:
    File "/usr/lib/python2.6/dist-packages/ganeti/cmdlib.py", line 11954, in CheckPrereq
      assert False

This is not perfect, but at least it shows better what the problem is.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>