Remind user to select a license

Currently, it is not obvious what's wrong when forgetting
to specify a license when choosing the third party origin
option in UploadWizard. This presumably because someone
mixed up "mwe-error-head" with "mwe-error-main".

Change-Id: I3477099d0eabd6e2fb5a90ee7e13ec6ac6396705
Merge "Remind user to select a license"