Downgrade some exception LOG messages in the ec2 API

Downgrades some LOG.exception messages to more appropriate error
or debug level instead. Tracebacks should only be produced where
they are really necessary. In two of the cases the error path is entirely
expected as a result from user error and should therefore only be
logged at debug level.

Change-Id: Ib8321836c622a30c37c97fa971cb9fcf406bd2f1
Merge "Downgrade some exception LOG messages in the ec2 API"