Plan/Share/Invite Collaborators function - more helpful error message when Email field is not filled in

[dsisu]

Please complete the following fields as applicable:

What version of the DMPRoadmap code are you running? (e.g. v2.2.0) 3.2.1

Expected behaviour: Under Plans/Share/Invite Collaborators

If Email field is not filled in before the user hits the Submit button, a more helpful error message should be issued. For example, the text 'You must enter an email address' could appear in red to the right of (or above, or below) the Email field.

Actual behaviour: If the Email field is not filled in, the user is presented with a University of Edinburgh page unavailable screen.

Steps to reproduce: Go to a plan of your own, select the tab Share and click Submit without filling in Email field.

[dsisu]

Also see issue https://github.com/DMPRoadmap/roadmap/issues/2246