Handle client disconnect during image upload

If a user does a ^C during a chunked image upload, eventlet.wsgi.server
will raise one of ValueError or IOError when trying to read the next
chunk.

Handle this common scenario by raising(logging) a HTTPBadRequest error rather
than HTTPInternalServerError.

Fixes bug 1196953

Change-Id: Ic88b142a40c548141be4b40a15f94b71603814e9