With MD5Sum checking being added, a concern was brought up that It was
a change with no possibility of reverting to the old behaviour.
This change adds the flag '--ignore-checksum' to the upload subcommand
allowing the checks to be turned off.
Changed occurrences of the magic string for a null md5 to use a descriptive
constant instead.
Updated Error messages generated when validation fails. They should now be more descriptive
and not output a literal newline sequence.
Change-Id: Id1756cbb6700bb7e38f0ee0e75bc535e37f777ed