Rename _check_image_size to _get_and_check_image_metadata

This method has functionality which doesn't seem to match what
it actually does (which is getting image metadata and performing
basic checks on that data).

Change-Id: I818fb5079105cca8acbdf464d47a92f7b1ebb398
Call format_message on InstanceTypeNotFound exception

Put the exception message from InstanceTypeNotFound into the
HTTPBadRequest rather than the exception itself.  Prevents a Circular
reference error during serialization later.

Bug 1175414

Change-Id: I4811e727311af670cd964a6d16ac5fce0fe3186e
Merge "Rename _check_image_size to _get_and_check_image_metadata"
Merge "Call format_message on InstanceTypeNotFound exception"