Further clarify 'too many values to unpack' error.

Many errors in yaml configurations cause ValueError to get thrown
with the message 'too many values to unpack.'  A previously reverted
change tried to handle all these situations and print an appropriate
message.

The current behavior of throwing the ValueError exception and exiting
is probably what we still want in these cases.  So instead of handling
the error, the code now checks for the exception at the top-most call
and displays appropriate messages in log.error and in the ctx.summary
data itself.

Fixes: 7510
Signed-off-by: Warren Usui <warren.usui@inktank.com>
Merge pull request #253 from ceph/wip-7510-wusui

Further clarify 'too many values to unpack' error.