get-default-creds failing to find default profile

[tlelson]

After generating an aws credentials file from an AD provider get-aws-creds returns an error that 'default profile could not be found'.

It seems that this is because the the get_mfa_serial_if_user raises an exception when it should just return false (having not found an mfa_serial)

Possibly related to #13 .

[tlelson]

trial this commit before closing