Return an error when the form is invalid
Return an error when the form is invalid

When an invalid POST request is made, the server was crashing with the folowing
error:

"""
ValueError: The view
dashboard_app.views.image_reports.views.image_chart_settings_update didn't
return an HttpResponse object. It returned None instead.
"""

Return HTTP 400 (bad Request) instead.

Change-Id: If33b65a39f7badfc57a56230dd9d7b6bcbb86a2c
(cherry picked from commit 6be471b3d4a3ff595d0815e2e56caf36d965bbf7)

Linaro/lava-server,Linaro/lava-server,Linaro/lava-server,Linaro/lava-server