Remove ImageTooLarge exception

InstanceTypeDiskTooSmall and ImageTooLarge are really two sides of the same
coin; an image is larger than the root_gb of the instance_type. Rather than
have two exceptions for this, this patch consolidates down to just
InstanceTypeDiskTooSmall.

Change-Id: I401205072c111a960beb2932c6c1889141ae03c3
Merge "Remove ImageTooLarge exception"