Fixed up BotoServerException's use of message

The BotoServerException class uses __getattr__() to check the message variable, but this is never going to work as it inherits from StandardError which defines the same variable.

This commit changes the default behaviour to use the message variable by default instead and add support for handling the now old variable name (error_message).

This change also means that the message variable will be populated by default which people generally expect when dealing with exceptions (it's currently not).