Log when image_size_cap has been exceeded

Add a log entry when an attempt to upload an image whose size is greater
than image_size_cap has been made.

Addresses bug 1154606.

Change-Id: I836cb2dad1c130c0df15d26a87229b60a6cc3033
Merge "Log when image_size_cap has been exceeded"
Merge remote-tracking branch 'm_src_repo/webob-issue' into build-temp