Change TaskNotFound traceback info to user-friendly message

In some common cases (like detecting absense of requested object),
CLI shows exception traceback instead of just a simple message
that "<something> is not found".

This traceback is redundant and confusing users - they think that
rally went into trouble, but all that actually happened is wrong
uuid specified.

The traceback must be shown only with `--debug' option explicitly
specified.

The unit test and functional test are both provided.

Change-Id: I32811974d88f414c4508dfed19a7c5dc5e80fd98
Closes-Bug: #1392354
Merge "Change TaskNotFound traceback info to user-friendly message"