Introduce InternalError EC2 error code

New InternalError exception with 'InternalError' ec2_code attribute was
introduced for the case of timeout when waiting for instance to stop
because there is no fitting existing exception.

InternalError is a valid EC2 error code. Unlike the other implemented
error codes, it's a server error with 500 status code. For that reason,
it's not listed as an expected error in the Executor and thus it's
treated as an unexpected error - it's logged as ERROR and returned
with 500 status code. That is a wanted behaviur because this shouldn't
ever happen and if it does, it should be visible.

Implements: blueprint ec2-error-codes

Change-Id: I11d25abfe46f5b3acdecb4fa2d31f4cec054cba1
Merge "Introduce InternalError EC2 error code"