More helpful error when signed in an accept invite

app/main/views/invites.py

@@ -27,7 +27,14 @@ def accept_invite(token):
     invited_user = invite_api_client.check_token(token)
 
     if not current_user.is_anonymous() and current_user.email_address != invited_user.email_address:
-        flash("You can't accept an invite for another person.")
+        flash("""
+            You’re signed in as {}.
+            This invite is for another email address.
+            <a href='{}'>Sign out</a> and click the link again to accept this invite.
+        """.format(
+            current_user.email_address,
+            url_for("main.sign_out")
+        ))
         abort(403)
 
     if invited_user.status == 'cancelled':

tests/app/main/views/test_accept_invite.py

@@ -284,7 +284,10 @@ def test_signed_in_existing_user_cannot_use_anothers_invite(app_,
             assert page.h1.string.strip() == '403'
             flash_banners = page.find_all('div', class_='banner-dangerous')
             assert len(flash_banners) == 1
-            assert flash_banners[0].text.strip() == "You can't accept an invite for another person."
+            banner_contents = flash_banners[0].text.strip()
+            assert "You’re signed in as test@user.gov.uk." in banner_contents
+            assert "This invite is for another email address." in banner_contents
+            assert "<a href='/sign-out'>Sign out</a> and click the link again to accept this invite." in banner_contents
             assert mock_accept_invite.call_count == 0