Output clear error message on invalid api version

Now if --os-image-api-version mistakenly contains a string then
you will get a ValueError

Example:
$ glance --os-image-api-version hui
ValueError: invalid literal for int() with base 10: 'hui'

This code correctly handles this situation, prints a warning
message and interrupts the execution of the client

Change-Id: I4733578adfc70bd57afd5f0d4d361c8ef689a381
Closes-bug: 1401197
Merge "Output clear error message on invalid api version"